dump: Use parasite file descriptors draining

Since we're able to simply drain file descriptors
we're interested in, just do that with parasite
help.

This changes the calling sequence a bit

 1) Collect file descriptors before parasite
    get intected the dumpee and remember them in
    local copy.

 2) Ask parasite to drain collected descriptrs
    into our space.

 3) Operate with file descriptors directly via fcntl
    calls and such.

Overall idea is to prepare ground for fowners
dumping which will be addressed in further patches.

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