signal-util: make -1 termination of ignore_signals() argument list unnecessary

signal-util: make -1 termination of ignore_signals() argument list unnecessary

Clean up ignore_signals() + default_signals() + sigaction_many() a bit:
make it unnecessary to explicitly terminate the signal list with -1.
Merge all three calls into a single function that is just called with
slightly different parameters. And eliminate an unnecessary extra
iteration in its inner for() loop.

No change in behaviour.