pgsql: Implement dblink_get_notify().

Log Message:
-----------
Implement  dblink_get_notify().

Adds the ability to retrieve async notifications using dblink,
via the addition of the function dblink_get_notify(). Original patch
by Marcus Kempe, suggestions by Tom Lane and Alvaro Herrera, patch
review and adjustments by Joe Conway.

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.82 -> r1.83)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.82&r2=1.83)
        dblink.h (r1.22 -> r1.23)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.h?r1=1.22&r2=1.23)
        dblink.sql.in (r1.18 -> r1.19)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.sql.in?r1=1.18&r2=1.19)
        uninstall_dblink.sql (r1.7 -> r1.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/uninstall_dblink.sql?r1=1.7&r2=1.8)
    pgsql/contrib/dblink/expected:
        dblink.out (r1.25 -> r1.26)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out?r1=1.25&r2=1.26)
    pgsql/contrib/dblink/sql:
        dblink.sql (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/sql/dblink.sql?r1=1.21&r2=1.22)
    pgsql/doc/src/sgml:
        dblink.sgml (r1.8 -> r1.9)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/dblink.sgml?r1=1.8&r2=1.9)