USBasp improvements

Hello!

I'm about to improve USBasp - add software control for ISP SCK speed (so I could 
place USBasp in a sealed case and stop worry about "Slow SCK" jumper). In fact, 
i've already (sort of) implemented that, just not ready to submit patches yet.

I was planning to use Avrdude's -B, bitclock command line switch for that (IMHO 
it fits), but here's a problem: AVR doesn't have very much control on hardware 
ISP speeds - just by factor of 2 - 750kHz, 375kHz, 187.5kHz ...

Unfortunately -B means bit clock period not frequency, and 375kHz for example, 
results in 2.66666(6)us period.

Suggestions?

Jurgis