http proxy: client: unix socket support

This allows the client stuff to understand that addresses beginning with '+'
represent unix sockets.

If the first character after the '+' is '@', it understands that the '@'
should be read as '\0', in order to use Linux "abstract namespace"
sockets.

Further the lws_parse_uri() helper is extended to understand the convention
that an address starting with + is a unix socket, and treats the socket
path as delimited by ':', eg

http://+/var/run/mysocket:/my/path

HTTP Proxy is updated to allow mounts to these unix socket paths.

Proxy connections go out on h1, but are dynamically translated to h1 or h2
on the incoming side.

Proxy usage of libhubbub is separated out... LWS_WITH_HTTP_PROXY is on by
default, and LWS_WITH_HUBBUB is off by default.