New default for "trusted_networks" = "lan". This will check peer addresses
against the addresses/netmasks of the interfaces to establish whether the peer
is local.
Fixes#1754
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.