Make sure DOS COMMAND(0) returns an upper-case drive letter
rtlib/gfxlib2: Fix remaining references to XKeycodeToKeysym

(see also: b6673c42)
Allegro example: Print spaces to clear previously printed numbers
Allegro Unix headers: Add more X11 #inclibs

At least they're needed for the liballeg.a static lib from Ubuntu 12.04's
liballegro4.2-dev package.