Switch various builtin functions to use pg_lsn instead of text.

The functions in slotfuncs.c don't exist in any released version,
but the changes to xlogfuncs.c represent backward-incompatibilities.
Per discussion, we're hoping that the queries using these functions
are few enough and simple enough that this won't cause too much
breakage for users.

Michael Paquier, reviewed by Andres Freund and further modified
by me.

Author: robertmhaas

doc/src/sgml/catalogs.sgml

@@ -5244,7 +5244,7 @@
 
      <row>
       <entry><structfield>restart_lsn</structfield></entry>
-      <entry><type>text</type></entry>
+      <entry><type>pg_lsn</type></entry>
       <entry></entry>
       <entry>The address (<literal>LSN</literal>) of oldest WAL which still
       might be required by the consumer of this slot and thus won't be

doc/src/sgml/func.sgml

@@ -15884,35 +15884,35 @@ SELECT set_config('log_statement_stats', 'off', false);
        <entry>
         <literal><function>pg_create_restore_point(<parameter>name</> <type>text</>)</function></literal>
         </entry>
-       <entry><type>text</type></entry>
+       <entry><type>pg_lsn</type></entry>
        <entry>Create a named point for performing restore (restricted to superusers)</entry>
       </row>
       <row>
        <entry>
         <literal><function>pg_current_xlog_insert_location()</function></literal>
         </entry>
-       <entry><type>text</type></entry>
+       <entry><type>pg_lsn</type></entry>
        <entry>Get current transaction log insert location</entry>
       </row>
       <row>
        <entry>
         <literal><function>pg_current_xlog_location()</function></literal>
         </entry>
-       <entry><type>text</type></entry>
+       <entry><type>pg_lsn</type></entry>
        <entry>Get current transaction log write location</entry>
       </row>
       <row>
        <entry>
         <literal><function>pg_start_backup(<parameter>label</> <type>text</> <optional>, <parameter>fast</> <type>boolean</> </optional>)</function></literal>
         </entry>
-       <entry><type>text</type></entry>
+       <entry><type>pg_lsn</type></entry>
        <entry>Prepare for performing on-line backup (restricted to superusers or replication roles)</entry>
       </row>
       <row>
        <entry>
         <literal><function>pg_stop_backup()</function></literal>
         </entry>
-       <entry><type>text</type></entry>
+       <entry><type>pg_lsn</type></entry>
        <entry>Finish performing on-line backup (restricted to superusers or replication roles)</entry>
       </row>
       <row>
@@ -15933,26 +15933,26 @@ SELECT set_config('log_statement_stats', 'off', false);
        <entry>
         <literal><function>pg_switch_xlog()</function></literal>
         </entry>
-       <entry><type>text</type></entry>
+       <entry><type>pg_lsn</type></entry>
        <entry>Force switch to a new transaction log file (restricted to superusers)</entry>
       </row>
       <row>
        <entry>
-        <literal><function>pg_xlogfile_name(<parameter>location</> <type>text</>)</function></literal>
+        <literal><function>pg_xlogfile_name(<parameter>location</> <type>pg_lsn</>)</function></literal>
         </entry>
        <entry><type>text</type></entry>
        <entry>Convert transaction log location string to file name</entry>
       </row>
       <row>
        <entry>
-        <literal><function>pg_xlogfile_name_offset(<parameter>location</> <type>text</>)</function></literal>
+        <literal><function>pg_xlogfile_name_offset(<parameter>location</> <type>pg_lsn</>)</function></literal>
         </entry>
        <entry><type>text</>, <type>integer</></entry>
        <entry>Convert transaction log location string to file name and decimal byte offset within file</entry>
       </row>
       <row>
        <entry>
-        <literal><function>pg_xlog_location_diff(<parameter>location</> <type>text</>, <parameter>location</> <type>text</>)</function></literal>
+        <literal><function>pg_xlog_location_diff(<parameter>location</> <type>pg_lsn</>, <parameter>location</> <type>pg_lsn</>)</function></literal>
        </entry>
        <entry><type>numeric</></entry>
        <entry>Calculate the difference between two transaction log locations</entry>
@@ -16107,7 +16107,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
        <entry>
         <literal><function>pg_last_xlog_receive_location()</function></literal>
         </entry>
-       <entry><type>text</type></entry>
+       <entry><type>pg_lsn</type></entry>
        <entry>Get last transaction log location received and synced to disk by
         streaming replication. While streaming replication is in progress
         this will increase monotonically. If recovery has completed this will
@@ -16121,7 +16121,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
        <entry>
         <literal><function>pg_last_xlog_replay_location()</function></literal>
         </entry>
-       <entry><type>text</type></entry>
+       <entry><type>pg_lsn</type></entry>
        <entry>Get last transaction log location replayed during recovery.
         If recovery is still in progress this will increase monotonically.
         If recovery has completed then this value will remain static at
@@ -16335,7 +16335,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
         <literal><function>pg_create_physical_replication_slot(<parameter>slotname</parameter> <type>text</type>, <parameter>plugin</parameter> <type>text</type>)</function></literal>
        </entry>
        <entry>
-        (<parameter>slotname</parameter> <type>text</type>, <parameter>xlog_position</parameter> <type>text</type>)
+        (<parameter>slotname</parameter> <type>text</type>, <parameter>xlog_position</parameter> <type>pg_lsn</type>)
        </entry>
        <entry>
         Creates a new physical replication slot named