function to convert Linux errno to DOS errno.
convert read/write etc.. routines to return 'correct' errcodes.
real memory alloc/dealloc/realloc functions.
add an exec routine that does what boot_dos does, but correctly..
Add more of the BIOS area routines.
fixup everything to be more modular (why?, I like modular .... .).
modify int21_names.h to use printf-like format strings and actually tell parameters and results.