restore: signals: get rid of nr passed as pointer

Get rid of nr passed as pointer and return the number
of signals prepared from both calls -- open_signal_image
and prepare_one_signal_queue.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
dump: signals: let signal_queue_entry = 0, if no signals to dump

In other languages(e.g. python) such messages will be gone after
serialization, because they don't store any useful data.
protobuf-c doesn't do that, so lets do that manually.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
restore: signals: add informative comments

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
protobuf: move enum nd_type inside net_device_entry

We already have TUN member at enum fd_types at fdinfo.proto.
It causes protoc(not protoc-c) to fail because of the name
collision due to the odd implementation of enums in C++.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
criu2json: add initial code

Should work.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>