knotd: remove forced stop from signal handler

The signals are blocked during server shutdown. Therefore the signal
handler could not be called twice.
knotd: setup signal at once place
knotd: fix previously broken signal blocking

The former mask used to block signals was empty and had no effect.
knotd: properly set signal blocking for new threads

Active signal mask is inherited by new threads. Therefore, control
signals used by the main thread had to be blocked at the moment, when
new threds are created.
knotd: reset umask() when starting
Merge remote-tracking branch 'origin/master' into improve-signal-handling
Merge branch 'improve-signal-handling'