common,w32: Fix handle_to_fd to match use of _open_osfhandle.

* common/exechelp-w32.c (handle_to_fd): Use intptr_t.
(gnupg_wait_processes): Fix to use pid_to_handle.

Both of original MinGW and MinGW-W64 use intptr_t for the first
argument of _open_osfhandle. So, intptr_t is better here.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>