scd: Use pipe to kick the loop on NetBSD.

* configure.ac (HAVE_PSELECT_NO_EINTR): New.
* scd/scdaemon.c (scd_kick_the_loop): Write to pipe.
(handle_connections): Use pipe.

On NetBSD, signal to the same process cannot unblock pselect,
with unknown reason. Use pipe instead, for such systems.

• GnuPG-bug-id: T3778
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>