common: New helper function gnupg_close_pipe.

* common/exechelp-posix.c (gnupg_close_pipe): New.
* common/exechelp-w32.c (gnupg_close_pipe): New.

This function is mainly for documentation purposes and should be used
along with gnupg_create_pipe.

• Signed-off-by: Werner Koch <wk@gnupg.org>