quit: don't call signal() if we're using sigaction()

At best, this is a no-op.

At worst, it might:
 - Clobber a signal handler someone registered after us
 - Overwrite the signal mask or flags
 - Cause unregistration to fail (sigaction() isn't guaranteed to return the exact pointer passed to signal())

(cherry picked from commit 6d99204a829879a3dbe975c6158228d07bfc1a0d)