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

Log Message:
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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)