Further fixes for translation of PrintScreen/SysRq.

The previous fix was tested mainly on 3 AT keyboards with USB adaptors where
it works. 1 USB keyboard doesn't translate Alt-PrintScreen, so the software
has to do it.

Reorganize a little to share some code and to not translate the unusual usb
scan code0x8a unless an Alt modified is set. Remove redundant check of Alt
modifiers. Translation now more clearly filters out Alt-PrintScreen before
the check.

The table of errors fixed in the previous commit had many bugs. Correct
table:

K_RAW Ctl-PrintScreen: E0-2A-E0-37 -> E0-37
K_RAW Alt-PrintScreen (with 4 comb. of Ctl/Shift): 79 -> 54
K_RAW Pause/Break (with 4 comb. of Alt/Shift): E0-46 -> E1-1D-45
K_CODE PrintScreen (with 4 comb. of Ctl/Shift): 54 -> 5c
K_CODE Alt-PrintScreen (with 4 comb. of Ctl/Shift): 7e -> 54
K_CODE Pause/Break (with 8 comb. of Ctl/Alt/Shift): 6c -> 68

That is 25 of 32 shift combinations for 2 keys fixed. All 16 combinations
were broken for K_CODE and thus also for K_XLATE.