Fix posix_spawn signal defaulting.

Add a new stress test, and fix the code to pass it. We need to ensure that
we reset signal handlers for caught signals before unblocking signals in
the child, we need to ensure that this happens even if you haven't passed
a pthread_spawn_attr_t, and we need to ensure that this happens if you
pass in an empty sigdefault set.

Bug: http://b/68707996
Test: ran tests
Change-Id: I348e9b17b1bdf221591da42c0ada133d98471d66