Open the exec link at fd restore stage as yet another service fd,
then pass it to restover via args and just call prctl on it.

This is good for several reasons -- the amount of code required for
this is less and opening files should better happen before we switch
to restorer (opening will be complex and it's MUCH easier to open all
we need in one place).
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>