ptrace: check that process is stopped by SIGTRAP

All other states are unexpected.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
parasie: use only one command for daemonizing

Signed-off-by: Andrey Vagin <avagin@openvz.org>
parasite: restore registers after each parasite command

Currenly we can restore blocking signals with help ptrace,
so if registers are restored after command, a process state
will not be corrupted if criu crashes.