In cases where it makes sense, use calloc() instead of malloc():
* When allocating large objects with many members, many for which
NULL/0 is a good default value.
* Arrays etc where we explicitly initialize to NULL anyway.
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.
