pgsql: Refer to max_wal_senders in a more consistent fashion.

Log Message:
-----------
Refer to max_wal_senders in a more consistent fashion.

The error message now makes explicit reference to the GUC that must be changed
to fix the problem, using wording suggested by Tom Lane.  Along the way,
rename the GUC from MaxWalSenders to max_wal_senders for consistency and
grep-ability.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xlog.c (r1.385 -> r1.386)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.385&r2=1.386)
    pgsql/src/backend/replication:
        walsender.c (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/walsender.c?r1=1.13&r2=1.14)
    pgsql/src/backend/utils/misc:
        guc.c (r1.545 -> r1.546)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.545&r2=1.546)
    pgsql/src/include/access:
        xlog.h (r1.104 -> r1.105)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h?r1=1.104&r2=1.105)