libwebsockets

unix domain: fix path name length

Unlike any other sockaddr variant it turns out when sockaddr_un reports its
sizeof() to connect() or listen(), it is trimmed to the used length of the
sun_path[] member not including any trailing 0x00.

Until now we worked fine, but our actual UDS paths have a large number of
trailing 0x00 (shown as @ in most tools).  Clients and servers can still
interoperate if they both have this broken name.

This patch fixes it to trim the sockaddr_un to the path length so the name
is as you would expect.