Draft: Recurse to the signal handler if it becomes stuck

This is only to allow CI pick kf5 as the target branch, not to merge at present

If some stage of the crash handler stuck, we may still be able to continue with others (eg.: emergency save function didn't work? we may still run DrKonqi. DrKonqi couldn't be started? we may get a crash dump by normal core handling). The default ALRM handling isn't conducive to that, so raise an artificial signal that we handle instead.

Also, harden the alarm mechanism against a barrage of crashes which would otherwise effectively disable it by calling alarm() indefinitely.