ptrace: wait when queued SIGSTOP signals will be handled (v2)

SIGSTOP can't be blocked, so we need to wait when it will be handled.

v2: don't duplicate the same code twice

Reported-by: Mr Jenkins
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>