userns: define protobuf messages for uid and gid maps

Normally, one of the first steps after creating a new user namespace is
to define the mappings used for the user and group IDs of the processes
that will be created in that namespace. This is done by writing mapping
information to the /proc/PID/uid_map and /proc/PID/gid_map files
corresponding to one of the processes in the user namespace.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>