Allow compile-time culling of verbprintf() calls

This introduces a new macro, `MAX_VERBOSE_LEVEL`, which culls all `verbprintf()` calls above that `verb_level` at compile time, so that they never make it into the compiled code at all. This allows you to separate the debug and production builds by making production builds have a 'verbosity cap' with what debug messages are available.

On x86 you can barely notice the performance difference from this, but on ARM (especially with old GCC), varargs stuff really sucks a lot, and just calling `verbprintf()` at all (even to return immediately) is actually very expensive.

I don't have any performance numbers for this one at the moment (gotta get access to the ARM board I was testing on again), but I can provide them if you want them as justification.