compel: Handier API for stopping tasks for infection

Now we have two routines one of which needs a callback for
proc parsing. This is complex, but needed by CRIU. For others
let's have a single "stop" call that would to everything.

travis-ci: success for compel: Contrinue improving library
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>