TAMU Modifications to xinvaders 2.0

1. Fixes
	- limit maxshots and maxvshots resources to MAXSHOTS and MAXVSHOTS,
            so don't run off end of static arrays!
	- moved BaseRec, Base, MAXSHOTS, MAXVSHOTS to vaders.h,
	    and eliminated redundant declarations in other files.
	- made whole "form" widgit accept default actions, so that
 	    the bottom right corner around buttons is not dead to input.
2. Enhancements
	- changed MAXSHOTS to 6, and made button to increment maxshots
	- made fire keys ('z' and ' ') and fire button (middle) 
            automatically repeat firing (within maxshots limit) while pressed. 
