Resynchronize ISO streams on playback (re)start, so the stream positions
are aligned immediately. This is better than relying on rate matching
to correct any offsets.
For ISO server sockets, the QOS struct from getsockopt contains values
with different meaning from ISO client socket. Get the values via DBus
instead, which is right in both cases.
Add factored out helper for ISO socket I/O.
ISO sockets need synchronization of writes and audio position for
different stream fds in the same isochronous group, and it's easier to
separate out the part that coordinates it.
