Merge branch 'improve-signal-handling' into 'master'

* improve-signal-handling:
  knotd: handle control interrupts atomically
  review: bool signal variables
  fdset: require pselect(), remove unsafe compat implementation
  knotd: reset umask() when starting
  knotd: properly set signal blocking for new threads
  knotd: fix previously broken signal blocking
  knotd: setup signal at once place
  knotd: remove forced stop from signal handler

Merge request !308