dump: Introduce the pre-dump action

With this action criu will seize tasks, grab all its memory into
page-pipes, rest dirty tracker and will then release them. Writing
the memory from page-pipes would occur after tasks are unfreezed
and thus the frozen time should become reasonably small.

When pre-dump is in action, the dirty tracking is forcedly turned
off as well as tasks are resumed afterwards, not killed, by default.

This is a prerequisite for iterative migration.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>