op_reg_common.h: Continue refactoring

op_reg_common.h: Continue refactoring

The new op_reg_common.h did not have in it all the things that made
sense for it to have, including some comment changes that I should have
made when I created it.

I also realized the the new mechanism of using shifts allowed
RXf_PMf_STD_PMMOD_SHIFT to actually control things, rather than be a
 #define that one had to remember to change if those things changed
independently.

Finally, I created a check so that adding bits without adding them to
RXf_PMf_COMPILETIME will force a compilation error.  (This came from the
school of hard knocks)