pgsql: Add GUC krb_server_hostname so the server hostname can be

Log Message:
-----------
Add GUC krb_server_hostname so the server hostname can be specified as
part of service principal.  If not set, any service principal matching
an entry in the keytab can be used.

NEW KERBEROS MATCHING BEHAVIOR FOR 8.1.

Todd Kover

Modified Files:
--------------
    pgsql/doc/src/sgml:
        runtime.sgml (r1.325 -> r1.326)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.325&r2=1.326)
    pgsql/src/backend/libpq:
        auth.c (r1.124 -> r1.125)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c.diff?r1=1.124&r2=1.125)
    pgsql/src/backend/utils/misc:
        guc.c (r1.264 -> r1.265)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.264&r2=1.265)
    pgsql/src/bin/psql:
        tab-complete.c (r1.130 -> r1.131)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/tab-complete.c.diff?r1=1.130&r2=1.131)
    pgsql/src/include/libpq:
        auth.h (r1.27 -> r1.28)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/auth.h.diff?r1=1.27&r2=1.28)