[redbean] Support unix domain sockets in Fetch

[mterron]

There are several context (in particular security sensitive ones) where a service should be exposed only over a unix socket and not over the network (like blackholed does). At the moment, connecting to these services from Redbean is more difficult than it could be with a little bit of syntax sugar.

It'd be great if Redbean's Fetch could accept a unix:// address and not just http and https instead of having to use the barebones unix.socket, unix.bind, unix.write and unix.read dance.