pgsql: Add parameter krb_realm used by GSSAPI, SSPI and Kerberos to

Log Message:
-----------
Add parameter krb_realm used by GSSAPI, SSPI and Kerberos
to validate the realm of the connecting user. By default
it's empty meaning no verification, which is the way
Kerberos authentication has traditionally worked in
PostgreSQL.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        client-auth.sgml (r1.101 -> r1.102)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/client-auth.sgml?r1=1.101&r2=1.102)
        config.sgml (r1.153 -> r1.154)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.153&r2=1.154)
    pgsql/src/backend/libpq:
        auth.c (r1.156 -> r1.157)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c?r1=1.156&r2=1.157)
    pgsql/src/backend/utils/misc:
        guc.c (r1.423 -> r1.424)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.423&r2=1.424)
        postgresql.conf.sample (r1.231 -> r1.232)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.231&r2=1.232)
    pgsql/src/include/libpq:
        auth.h (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/auth.h?r1=1.33&r2=1.34)