Since this struct only contained function pointers, make all modules
export those functions directly.
The plugin manager now defines a module interface struct, and fills it
it by dlsym:ing the functions that used to be in module_info.
All modules are expected to handle a call to content() after having
been instantiated.
I.e. modules *cannot* even expect run() to have started running.
Previously we allowed it on the bar, and on all particles. Now we also
allow it on all modules.
This allows us to specify a "default" font/foreground on a per-module
basis, having it applied to all the modules particles.
Warn if volume and/or muted state is inconsistent (as we only expose a
single volume/muted state).
Also, don't query for current volume if max == 0 (i.e. typically a
digital output).
Finally, make absolutely sure that volume min is really less (or
equal) to volume max, and that the current volume level is between min
and max.
When snd_mixer_selem_get_*() fails, the values were left
uninitialized, trigger a valgrind warning.
In addition to fixing this, the error(s) are now logged (but only as
warnings, as they are not fatal).
