Back-patch some minor bug fixes in GUC code.

In 9.4, fix a 9.4.1 regression that allowed multiple entries for a
PGC_POSTMASTER variable to cause bogus complaints in the postmaster log.
(The issue here was that commit bf007a2 unintentionally reverted
3e3f659, which suppressed any duplicate entries within
ParseConfigFp.  Back-patch the reimplementation just made in HEAD, which
makes use of an "ignore" field to prevent application of superseded items.)

Add missed failure check in AlterSystemSetConfigFile().  We don't really
expect ParseConfigFp() to fail, but that's not an excuse for not checking.

In both 9.3 and 9.4, remove mistaken assignment to ConfigFileLineno that
caused line counting after an include_dir directive to be completely wrong.

Author: tglsfdc

src/backend/utils/misc/guc-file.l

@@ -587,7 +587,6 @@ ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
 								 head_p, tail_p))
 				OK = false;
 			yy_switch_to_buffer(lex_buffer);
-			ConfigFileLineno = save_ConfigFileLineno;
 			pfree(opt_name);
 			pfree(opt_value);
 		}