CRON: restore and fix deleted test
s/INT_ILLEGAL_OPCODE/INT_ILLEGAL_INSTRUCTION/g
introduce "illegal" instruction
move MD-related check to md body
move user-legalness check to op bodies
fixed SINT/SIND, changed SIT/CIT/SIL/SIU handling
s/cpu_mod/cpu_mod_active/g
int_timer/int_extra cleanup
register_op fix, CRON move/fix
interrupt naming fix
illegal cron handled as illegal
tests: 17-bit byte addressing in user mode