Refactor main loop

[mfld-fr]

Tried to make the main loop more readable:

• debug procedure moved out
• command line processing moved out
• avoid breaking the program path with exit()

[ghaerr]

Quick first review: very nice, exactly what is needed.

Lets hold overnight before commit. I'm deep inside #31 (^C terminal exit) and it's proving to be very, very strange. I have Linux VM running. However, I found in debugging that that we have another small problem: the atexit call in char-stdio.c ends up calling char_term multiple times when exit is called explicitly to end EMU86. Now, in this version, exit is removed and return is used from main, which will call exit in crt0.S. I suggest calling _exit in main to fix this. Or this can be done separately with my fix, when I finally figure it out!

[ghaerr]

@mfld-fr: In emu-main.c line 611, suggest changing 'return' to '_exit', and merge. This will fix multiple calls to char_term caused by atexit in char-stdio.c::char_init. The atexit is currently necessary because EMU86 may call exit at various times, and console could be left in raw mode. Better fix would be to replace all 'exit' calls with 'emu_term' which calls all image_close/con_term/serial_term and then 'exit'.

After merge, I will test Emscripten/browser port.

Thank you!

[ghaerr]

Update: I have tested Emscripten/browser support with this PR included and it works! However, there is a small tuneup that needs to be made for speed in timer-elks.c with SDL or Emscripten. I will submit tuning PR and instructions for Emscripten build after merge.

[mfld-fr]

Suggestion applied. Thanks for PR testing !