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
fdset: require pselect(), remove unsafe compat implementation
review: bool signal variables
knotd: handle control interrupts atomically
tests-extra: reduce sleep (2 sec) interval between lsof calls

The previous value (4 sec) extended ixfr/from_differences execution
time by 40 minutes!
Coverage: update new directory structure
tests-extra: CNAME loop + fixed basic/query test.

- Test response with a long CNAME loop.
- Some queries in basic/query test did not have the zone name
  appended, resulting in bogus answers (REFUSED). Not visible,
  because the responses are only compared to ones from Bind.

TODO: Comparison with Bind should be replaced by checks for
      particular requirements for the response.
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