pgsql: Minor API cleanup for async notifications: we can only register

Log Message:
-----------
Minor API cleanup for async notifications: we can only register the
current backend in pg_listener, so there is little point in making
the PID to register part of async.c's public API. Other minor tweaks.

Modified Files:
--------------
    pgsql/src/backend/commands:
        async.c (r1.124 -> r1.125)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/async.c.diff?r1=1.124&r2=1.125)
    pgsql/src/backend/tcop:
        utility.c (r1.243 -> r1.244)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c.diff?r1=1.243&r2=1.244)
    pgsql/src/include/commands:
        async.h (r1.28 -> r1.29)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/async.h.diff?r1=1.28&r2=1.29)