Daniel Verkamp: COMBOOT Support

Allow COMBOOT images to return via INT 20h - save the (far) return address during exec, then save the old stack address in the PSP, so in INT 20h handler, just change back to old stack and do RETF

Handle other termination API functions (INT 21h, AH=00h and AH=4Ch) - call INT 20h handler

Handle INT 21h, AH=09h (Write DOS String to Console) and AH=30h (Check DOS Version)

Add beginning of SYSLINUX AX=0001h (get version) and AX=0003h (run command)

Run command function was looping on itself (comboot executable ran again when “boot” executed?), so did similar “unregister image while running” like image/script.c

Meeting with mdc