scd: Fix for GNU/Linux suspend/resume.

* configure.ac (require_pipe_to_unblock_pselect): Default is "yes".
* scd/scdaemon.c (scd_kick_the_loop): Minor clean up.

Normally SIGCONT or SIGUSR2 works for unblocking pselect. But on my
machine with GNU/Linux, when a machine is suspend/resume-ed, pselect
keeps blocked, while signal itself is delivered.

It's better to use pipe.

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