pgsql: Make the pg_stat_activity view call a SRF

Log Message:
-----------
Make the pg_stat_activity view call a SRF (pg_stat_get_activity())
instead of calling a bunch of individual functions.

This function can also be called directly, taking a PID as an argument, to
return only the data for a single PID.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        monitoring.sgml (r1.57 -> r1.58)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml?r1=1.57&r2=1.58)
    pgsql/src/backend/catalog:
        system_views.sql (r1.49 -> r1.50)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/system_views.sql?r1=1.49&r2=1.50)
    pgsql/src/backend/utils/adt:
        pgstatfuncs.c (r1.49 -> r1.50)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pgstatfuncs.c?r1=1.49&r2=1.50)
    pgsql/src/include/catalog:
        catversion.h (r1.455 -> r1.456)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.455&r2=1.456)
        pg_proc.h (r1.496 -> r1.497)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.496&r2=1.497)
    pgsql/src/test/regress/expected:
        rules.out (r1.136 -> r1.137)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/rules.out?r1=1.136&r2=1.137)