files: Rework send/recv-fds to be more generic

Remove getting opts from descriptors out from scm engine,
this stuff is pure criu thing, so make it collect the data.

The tricky change here is that parasite code needs memory
to keep fd_opts on. The memory is taken from parasite args
region, which is now bigger than it used to be. But that's
not a big deal, as previously this space was allocated on
the parasite stack (!, but with smaller chunks).

On the other hand, now we have one memcpy less, as opts are
put directly into the destination buffer.

travis-ci: success for files: Rework send/recv-fds to be more generic
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>