Fixed the bug with empty main thread ID in output
Changed sa_sighandler to sa_sigaction in signals registration

According to man signal, the old approach worked because of some
magic luck (see also https://github.com/vmarkovtsev/DeathHandler/issues/6)
Added SA_ONSTACK for handling stack overflow

See https://github.com/vmarkovtsev/DeathHandler/issues/6
Added SIGFPE handler

See https://github.com/vmarkovtsev/DeathHandler/issues/7