signals: fix SIGINT blocking flaw causing inconsistent metadata

signals: fix SIGINT blocking flaw causing inconsistent metadata

SIGINT isn't blocked properly after a sigint_allow(),
sigint_restore() cycle leading to illicit interruptable
metadata updates.  These can leave corrupted metadata behind.

Issues addressed in this commit:

sigint_allow() fails to set _oldmasked[] members properly due
to an offset by one bug on indexing the members of the array.

It bails out prematurely comparing to MAX_SIGINTS causing nesting
depths to be one less than MAX_SIGINTS.  Fix the comparision.

Correct the related comparison flaw in sigint_restore().

Initialize all sig_atomic_t variables consequently.

Resolves: rhbz1440766