scd: signal mask should be set just after npth_init.

* scd/scdaemon.c (setup_signal_mask): New.
(main): Call setup_signal_mask.
(handle_connections): Remove signal mask setup.

For new thread, signal mask is inherited by thread creation.
Thus, it is best to setup signal mask just after npth_init.

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