SPA_MEMBER is misleading, all we're doing here is pointer+offset and a
type-casting the result. Rename to SPA_PTROFF which is more expressive (and
has the same number of characters so we don't need to re-indent).
The simple protocol listens on a socket, creates a new pw_client for
each connected client and reads/writes raw audio bytes on the
socket. Incomming bytes are copied to a playback stream, outgoing
bytes are copied from a capture pw_stream.
Fixes#954
