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.

Same as with previous patch -- at the end there will be only
two trap commands, so this is OK.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>