Re-evaluate thread' signal mask after ptracestop().

The stop drops process lock, which allows the signal mask to be
changed and our selected signal might become blocked, i.e. should be
returned to the process queue instead of delivery.

Also, for the existing check of the process no longer having an
attached debugger, we should not loose the signal, but requeue it.

Reported and tested by: bdrewery
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week