net: host/port string convention

net: host/port string convention

This updates the implementation of host_and_port() in such a way that its
output can again be used as input of parse_url():

 * the separator for host and port is now ':' instead of '#';
 * port numbers (services) are now always represented as numbers
   (reverting an earlier change);
 * IPv6 addresses are enclosed in square brackets.

This convention makes it easier to copy&paste output from paraslash commands
(including scripting), and to store target information in a format that is
easy to resolve/reuse at a later stage.