pgsql: Add support for using SSL client certificates to authenticate to

Log Message:
-----------
Add support for using SSL client certificates to authenticate to the
database (only for SSL connections, obviously).

Modified Files:
--------------
    pgsql/doc/src/sgml:
        client-auth.sgml (r1.111 -> r1.112)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/client-auth.sgml?r1=1.111&r2=1.112)
        runtime.sgml (r1.421 -> r1.422)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml?r1=1.421&r2=1.422)
    pgsql/src/backend/libpq:
        auth.c (r1.172 -> r1.173)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c?r1=1.172&r2=1.173)
        hba.c (r1.173 -> r1.174)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c?r1=1.173&r2=1.174)
        pg_hba.conf.sample (r1.65 -> r1.66)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pg_hba.conf.sample?r1=1.65&r2=1.66)
    pgsql/src/include/libpq:
        hba.h (r1.52 -> r1.53)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/hba.h?r1=1.52&r2=1.53)