daemon: support AF_UNIX sockets in the http module

There's no support for the "dns" and "tls" kinds (for now).

Tested briefly:

modules = { 'http' }
net.listen('/tmp/kresd-socket', nil, { kind = 'webmgmt' })

socat TCP-LISTEN:5555,reuseaddr,fork UNIX-CLIENT:/tmp/kresd-socket
xdg-open http://127.0.0.1:5555