Add Linux key tables to system_linux. (dm)
The Linux screen driver's implementation of PASSXT and PASSAT should use system_linux's key tables. (dm)
For now, LinuxKeyCode can be 8 bits (rather than 16). (dm)
Define (and use) constants for the XT/AT scan code modifiers. (dm)
Remove more hard-coded scan code translation from the Linux screen driver. (dm)