The old name was bad because port is not listening, rather *something* is
listening on the port.  The new name is better because we can extend it
like

    listening_on -u UDP_PORT
    listening_on -s UNIX_SOCKET
    listening_on -a 127.0.0.1 -t 80
    ...

without losing much readability.

This uses even other function __cut_stack to identify two function
names: $caller is the first function on the stack to be be outside
API and is interesting for user so can be used in pretty printer
(typically for debug output).  $internal is name of the wrapper
from this API; this is used to choose the right pretty printer
(debug, warn...).

Make it clear that the assignment must happen, otherwise we might easily
end up wit empty src or mixup between local and caller's local.

For some strange reason, the tee pipe did not work unless stuffed by cat
(all the output pipes remained empty)

(et voila,) tee supports more than one target

For example, in 'fc20', these will match 'fc' and '20', respectively.

Things have grown and some parts of layout now make more sense being in different module

Colorized output should bear roughly the same info as plain text even
after stripping color chars (e.g. by c/p)

Now it's getting serious ;)