pgsql: Make krb_realm and krb_server_hostname be pg_hba options only,

Log Message:
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Make krb_realm and krb_server_hostname be pg_hba options only, and remove
their GUCs.

In passing, noted that the pg_hba options for krb5 authentication weren't
listed at all - so add this.

Modified Files:
--------------
pgsql/doc/src/sgml:
client-auth.sgml (r1.117 -> r1.118)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/client-auth.sgml?r1=1.117&r2=1.118)
config.sgml (r1.203 -> r1.204)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.203&r2=1.204)
pgsql/src/backend/libpq:
auth.c (r1.177 -> r1.178)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c?r1=1.177&r2=1.178)
pgsql/src/backend/utils/misc:
guc.c (r1.491 -> r1.492)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.491&r2=1.492)
postgresql.conf.sample (r1.250 -> r1.251)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.250&r2=1.251)