pgsql: Fix some inconsistent quoting of wal_level values in messages

Log Message:
-----------
Fix some inconsistent quoting of wal_level values in messages

When referring to postgresql.conf syntax, then it's without quotes
(wal_level=archive); in narrative it's with double quotes. But never
single quotes.

Modified Files:
--------------
pgsql/src/backend/access/transam:
xlog.c (r1.416 -> r1.417)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.416&r2=1.417)
pgsql/src/backend/postmaster:
postmaster.c (r1.610 -> r1.611)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c?r1=1.610&r2=1.611)
pgsql/src/backend/replication:
walsender.c (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/walsender.c?r1=1.23&r2=1.24)