Introduce feature check via RPC

There are still systems which do not support dirty memory tracking.
This offers an interface to query the dirty memory tracking availability
via the new feature check RPC.

This is in preparation of a p.haul change which will use this RPC
interface to automatically detect if pre-dumps should be executed
or not.

This change introduces an additional optional field in the
criu_request and criu_response message (features) which is a
'criu_features' message.

Right now only the check for the memory tracking feature is supported
in the message 'criu_features':
	optional bool                   mem_track       = 1;

v2: Instead of checking for memory tracking only, provide a generic
    interface to check for arbitrary features.

v3: Do not use bitfields for feature encoding but protobuf optional
    message parameters.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>