security: Introduce (rather basic) security restrictions for C/R

Right now we have an ability to launch the C/R service from root
and execure dump requests from unpriviledged users. Not to be bad
guys, we deny dumping tasks belonging to user, that cannot be
"watched" (traced, read /proc, etc.) by the dumper.

In the future we will use this "engine" when launched with suid
bit, and (probably) will have more sophisticated policy.

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