accept new-style connection hints

Accept TCP connection hints in the new Twisted Endpoints syntax (e.g.
"tcp:host=127.0.0.1:port=9999").

Also, be more picky about what we take to be an old-style host:port connection
hint (for example, accept only a certain class of characters in hostnames, and
accept only port numbers that are 1 to 5 digits long).

If a connection hint is neither a new-style TCP Endpoint string, nor matches
the (now tighter) old-style host:port string, then it will be ignored. (This
makes it easy for future versions of foolscap to put other forms of connection
hints into furls, knowing that foolscap 0.7 will ignore them.)

Also, we no longer require ignored connection hints to contain a ':'.