A little reorganisation.
Added words `file-open`, `file-close` and `errno`.

file_open() and file_close() now set errno to 0 after success.
Introduced macros PROC1 and PROC2 for procedure-like words.
Reorganised code a bit and made it more compact.
Reconstruced explanations of CODE() macros that were erroneously deleted.
Bug fix in `file-open`.
mind can now interpret other files than just "init.mind".
The automatic tests are now done in the file tests.mind.