sd-rtnl: handle empty multi-part message from the kernel

sd-rtnl: handle empty multi-part message from the kernel

We strips out NLMSG_DONE piece from a multi-part message adding into the
receive queue only the messages containing actual data.

If we send a request to the kernel for getting the forwarding database table (just an example),
the response will be a multi-part message like below:
1. FDB entry 1;
2. FDB entry 2;
3. NLMSG_DONE;

We strip out "3. NLMSG_DONE;" part and places into the receive queue a pointer to
"1. FDB entry 1; 2. FDB entry 2".

But if the FDB table is empty, the respose from the kernel will look like below:
1. NLMSG_DONE;

We strip out "1. NLMSG_DONE;" part and since there is no actual data got, it continues
waiting until reaching timeout.

Therefore, a call to "sd_rtnl_call" to send and wait for a response from kernel will exit
with timeout which is interpreted as error in communication.

This patch puts the NLMSG_DONE message on the receive queue if it ends an empty multi-part
message. This situation is detected in sd_rtnl_call() and in the callback code and NULL is
returned to the caller instead.

[tomegun:
  - added/reworded commit message
  - extend the same support to sd_rtnl_call_async()
  - drop debug logging from library, we only do this if something is really wrong, but an
    empty multi-part message is perfectly normal
  - modernize the code we touch whilst we are at it]