Fix handling of NumLock

Find out which modifier key is used by Num_Lock (`x_numlock_mod()`) and
register grabs for both variants: modifier present/not present.

This fixes issue #106.

Possible enhancement: Do not run x_numlock_mod() each time, but only on initialization. Possible downside of this enhancement: xmodmap changes would not be seen (but how often do they occur?). As I don't know about the right structure/place to put this, I haven't done this 'one-time-only'.