socket: Add vectored I/O support for receiving on sockets

socket: Add vectored I/O support for receiving on sockets

Replace the recv() API with a recv_messages() API, which supports
receiving multiple messages, each with multiple buffers rather than a
single monolithic buffer.

This doesn’t break API, as the socket API is not exposed outside
libnice. It does introduce a new struct: NiceInputMessage, which is
analogous to struct mmsghdr.

This includes updates to the test-bsd test to cover the changed API.