common: Use gnupg_spawn_process_fd to invoke gpg-agent/dirmngr.

* common/asshelp.c (start_new_gpg_agent): Call gnupg_spawn_process_fd
and gnupg_wait_process.
(start_new_dirmngr): Likewise.

With --daemon option, gpg-agent/dirmngr detaches by itself.

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