pgsql: Control client certificate requesting with the pg_hba option

Log Message:
-----------
Control client certificate requesting with the pg_hba option "clientcert"
instead of just relying on the root certificate file to be present.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        runtime.sgml (r1.420 -> r1.421)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml?r1=1.420&r2=1.421)
    pgsql/src/backend/libpq:
        auth.c (r1.171 -> r1.172)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c?r1=1.171&r2=1.172)
        be-secure.c (r1.85 -> r1.86)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/be-secure.c?r1=1.85&r2=1.86)
        hba.c (r1.172 -> r1.173)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c?r1=1.172&r2=1.173)
    pgsql/src/include/libpq:
        hba.h (r1.51 -> r1.52)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/hba.h?r1=1.51&r2=1.52)
        libpq.h (r1.69 -> r1.70)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq.h?r1=1.69&r2=1.70)