Guess is now section head + FFOO_INI_SUFFIX, to allow for customized
config trees.

The (not sure if )written convention is that module's variable names
should be composed as

    FFOO_BAR_BAZ

for variable bearing *baz* in module named *bar*.

The stack now supports passing down parameters to yum, so there's no
need to another case.

Allow passing doen yum arguments like `--enablerepo="*"`

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...).