signal-util: use common definitions for ignore + default "struct sigaction"

signal-util: use common definitions for ignore + default "struct sigaction"

We use this at various places, let's unify this in one global constant.

This changes flags in crash-handler.c in a tiny irrelevant way: we ask
syscalls to be continued on signal arrival, which we previously didn't.
But that shouldn't change anything, the only thing we'll do in the
relevant process is call raise(), and that's it, hence there definitely
are no syscalls to restart or not to restart.