crtoools: add support of stopped tasks (v2)

Currently we catch processes on the exit point from sigreturn.
If a task must be restored in the stopped state, we can send SIGSTOP
before detaching from it.

v2: add more descriptive comment about skipping SIGSTOP in ptrace.c
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>