pgsql: When we're restricting who can connect, don't allow new

Log Message:
-----------
When we're restricting who can connect, don't allow new walsenders.

Normal superuser processes are allowed to connect even when the database
system is shutting down, or when fewer than superuser_reserved_connection
slots remain.  This is intended to make sure an administrator can log in
and troubleshoot, so don't extend these same courtesies to users connecting
for replication.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.269 -> r1.270)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.269&r2=1.270)
    pgsql/src/backend/utils/init:
        postinit.c (r1.211 -> r1.212)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c?r1=1.211&r2=1.212)