Currently parasite is loaded using the map_files dir,
which is guarged with CAP_SYS_ADMIN by default (which
is dropped in 4.2 series). So lets do a deal -- try
to use memfd interface first (which has been introduced
in 4.0 kernel series) and if we fail then switch to old
map_files interface.

With time all users are switched to new kernels so
memfd gonna be primary interface.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Looks-good-to-me: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Looks-good-to-me: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>