pgsql: Add pg_terminate_backend() to allow terminating only a single

Log Message:
-----------
Add pg_terminate_backend() to allow terminating only a single session.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.430 -> r1.431)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.430&r2=1.431)
        runtime.sgml (r1.411 -> r1.412)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml?r1=1.411&r2=1.412)
    pgsql/src/backend/tcop:
        postgres.c (r1.548 -> r1.549)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c?r1=1.548&r2=1.549)
    pgsql/src/backend/utils/adt:
        misc.c (r1.59 -> r1.60)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/misc.c?r1=1.59&r2=1.60)
    pgsql/src/include/catalog:
        pg_proc.h (r1.489 -> r1.490)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.489&r2=1.490)
    pgsql/src/include/storage:
        proc.h (r1.104 -> r1.105)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/proc.h?r1=1.104&r2=1.105)
    pgsql/src/include/utils:
        builtins.h (r1.312 -> r1.313)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.312&r2=1.313)