Extend the RPC feature check functionality to also test for lazy-pages
support. This does not check for certain UFFD features (yet). Right now
it only checks if kerndat_uffd() returns non-zero.

The RPC response is now transmitted from the forked process instead of
encoding all the results into the return code. The parent RPC process
now only sends an RPC message in the case of a failure.
Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>