Refactored UDP socket_send_messages to use g_socket_send_messages

This request tries to address what I introduced in #117 (closed).

First of all, it collapses the static socket_send_message and socket_send_messages methods in udp-bsd.c in a single method, which means there now only is socket_send_messages (which is the one that is exported to the libnice core). In fact, they would have shared a lot of code anyway, and so the method internally decides which GIO socket primitive to use depending on the number of messages it's trying to push:

• if n_messages is 1, then it uses g_socket_send_message, which means the method is equivalent to what the code did before in socket_send_message;
• if n_messages higher than 1, it means we're trying to send more than one message at the same time, and so we prepare a GOutputMessage array and pass it to g_socket_send_messages, which as I said in #117 (closed) internally uses sendmmsg if the platform supports it.

Everything should be exactly the same in term of APIs and what is returned: it's just an internal change on how to deal with multiple messages, so hopefully it doesn't break anything. The tests seem to work fine as well, with the exception of test-bsd (but that may be because I'm on Linux and not BSD?). In particular, I've checked test-send-recv, since I've noticed it does use nice_agent_send_messages_nonblocking to send a bunch of messages together, and that seems to work as expected.

That said, I've added the WIP: prefix to the merge request to be sure I'm not doing anything wrong, since I'm not 100% familiar on the internals. Next step on my end is to work on a branch in Janus to use nice_agent_send_messages_nonblocking instead of nice_agent_send, and then do some performance assessment to verify whether or not this patch helps.

Looking forward to your feedback!