Let's make sure our poll() calls don't get interrupted where they shouldn't (SIGALRM, ...), but allow them to be
interrupted where they should (SIGINT, ...).
Fixes#1965
This ports a lot of manual code over to sigprocmask_many() and friends.
Also, we now consistly check for sigprocmask() failures with
assert_se(), since the call cannot realistically fail unless there's a
programming error.
Also encloses a few sd_event_add_signal() calls with (void) when we
ignore the return values for it knowingly.