dump: Introduce "post-dump" script action

This is usefull if one needs to do some final action before checkpoint
is complete. For example in case of online migration one may provide
a script which would check the restore procedure on remote note
ended without errors, thus the script returns zero code and criu
simply kills running instance of application.

In turn, if migration failed, the script can return nonzero code
and criu won't kill the application but continue its execution
instead.

https://bugzilla.openvz.org/show_bug.cgi?id=2583



Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>