Rearranged user input (quit) coping order.

Quitting request (q) moved to the end of simple actions, just before
commands. Added 'quit' and 'exit' as quitting requests.
Changed also erroneous return value on quit: must be 0 .
Implemented return to same-current task on system call or task error.

Tidying up the program flow: commands.
Added some code comments. Else instead of elsif in system() call.
Minor code comments format.
Minor code comments format.