parasite: stop task on exit from a specified syscall

This patch adds nothing new, just splits the existant function.

Currently a parasite stopped on sigreturn for unmaping a parasite blob.
The same scheme will be used for restorer blob and this function will be
used to stop on exit from the munmap syscall.

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