Rename version protobuf RPC members

In rpc.proto the interface to query the CRIU version number uses major
and minor as keywords. This creates errors when using the RPC
definitions with C++: https://github.com/checkpoint-restore/criu/issues/625



In this commit the fields are renamed from major to major_number and
from minor to minor_number.

For existing programs using the RPC protobuf definition this should be a
transparent change. Only for programs importing the latest rpc.proto it
will require code changes.

Signed-off-by: Adrian Reber <areber@redhat.com>