Emit warning when ignoring return value for certain functions
Allocate running modules separately, improve handling stumbled modules

A module is no longer simply copied from the `modules_enabled` list, but
allocated separately. This should simplify having multiple instances of
the same module, which makes karuibar in general more modular.

If a module stumbles, it is term()inated and all callbacks are set to
NULL. Hence there is no reason to keep the `stumbled` indicator for a
module.
Add field (pointer) for custom module data, compress&simplify

A future goal is to be able to enable modules multiple times but with
different options. So for instance if one would like to track battery
information for more than one battery, having two separate
configurations would be handy. This is the first step to modularise
things further.

Also, simplify a few snippets, and make [cpu] and [battery] more narrow
in their display.