socket() with SOCK_NONBLOCK (O_NONBLOCK) seems not to be possible on MacOS, it
yields 'Protocol wrong type for socket'. Switch to using fcntl() and O_NONBLOCK
instead, hopefully works better cross-platform.
Closes#1644
Using worker threads instead of httpd threads means that we, not libevent,
decide which requests get handled by which threads. This means that we can
make sure blocking requests (e.g. volume changes) don't get in the way of
realtime(ish) stuff like mp3 streaming.
Includes refactor of httpd_stream_file() since it was a bit of a monster.
