core: introduce helper tt_sigaction

The problem is that even if we block all signals on all
threads except the main thread, the signals still can be
delivered to other threads (#7206). And another problem
is that user can spawn his own thread and not block
signals.

That is why the patch introduces tt_sigaction function that
guarantees that all signals will be handled only by the main
thread. We use this helper in clock_lowres module.
This is supposed to solve the problem, described in #7408.

NO_CHANGELOG=internal
NO_DOC=internal