Fix incorrect cursor location in magit-key-mode

After setting an argument value, point would be in the wrong spot. Fix
this by saving which option point is at before redrawing, then jumping
to that option after redrawing. Add a new function
magit-key-mode-build-exec-point-alist to help with this.
Fix compiler warnings
Merge commit '92fdb63'

Conflicts:
	magit-key-mode.el