libwebsockets

http: server: support HEAD via mount

Until now we parse HEAD requests but don't properly fulfil them.

This adds enough that if the request pointed to a valid mount,
it will send the headers and complete the transaction without
sending the body.

Test with

$ (echo -n -e "GET / HTTP/1.0\r\nHost: default\r\n\r\n"; sleep 2) | nc  127.0.0.1 7681