[RP] fixing up Mode_switch

[shawn]

So I've made some changes to implement altgr properly. But X is doing
funny stuff. I have Mode_switch attached to Mod3, but when I press my
Mode_switch key rather than the 2<<5 bit flipping, the 2<<13 bit gets
flipped. There's no #define for this either. It also doesn't matter
what mod I put it on, I always get 2<<13. 

I've never been too skilled at remapping keys under X, so maybe
there's something I've forgotten. I've commented out a line in input.c
that checks the 2<<13th bit instead of the mode_switch_mask and all's
well, but dirty and incorrect as far as I can tell.

As a side-effect it now possible to detect meta, alt, hyper, and super
modifiers with rp_modifier_info.

Anyway, I sleep now.