Some hw breakpoint fixes
Some hw breakpoint fixes

Enable usage of dr1-dr3.  Clear control bits when a breakpoint
is disabled. Use DR_LEN_1 instead of DR_LEN_8 so breakpoint work on
unaligned adresses.

Fixes #51.

joeshaw/delve,beni55/delve,w4ngyi/delve,glycerine/delve,aarzilli/delve,torfuzx/delve,derekparker/delve,skyportsystems/delve,torfuzx/delve,visualfc/delve,alexbrainman/delve,sinuos/delve,alexbrainman/delve,tylerb/delve,ggndnn/delve,tylerb/delve,dr2chase/delve,yigen520/delve,sinuos/delve,aperera/delve,go-delve/delve,dlsniper/delve,yigen520/delve,waterway/delve,skyportsystems/delve,dlsniper/delve,aarzilli/delve,ignatov/delve,joeshaw/delve,glycerine/delve,derekparker/delve,w4ngyi/delve,beni55/delve,jeff235255/delve,go-delve/delve,ggndnn/delve,dr2chase/delve,derekparker/delve,alexbrainman/delve,ironcladlou/delve,go-delve/delve,firebitsbr/delve,firebitsbr/delve,aarzilli/delve,dlsniper/delve,go-delve/delve,ironcladlou/delve,jeffallen/delve,dullgiulio/delve,jeff235255/delve,ggndnn/delve,go-delve/delve,lukehoban/delve,visualfc/delve,aarzilli/delve,lukehoban/delve,ignatov/delve,ignatov/delve,visualfc/delve,dullgiulio/delve,waterway/delve,aperera/delve,dr2chase/delve,jeffallen/delve