Tail and Head commands reimplementation with an helper function and a new error type. + Updating documentation (mans files and Manual.txt file).

[mdaadoun]

• A new error call added : Missing arguments
• Replaced hardcoded error from pexecc, fpexecc and mann with this new error call.
• Reimplementation of the head command while adding the tail command. Because they use most of the same logic, created a subcommand headtail to use by both of them.
• Improved the documentation (in comments and man files) for errors, commands and the new helper function, future with generation of manual from python docstring ? (PEP 257)
• Added an helper function, get_valid_options, to deal with parameters passed with each command who need it. format '-abcd'.
• Created a space fn to gather thoses common helper functions.
• Improved the Manual.txt file with errors and helper function to help for development while removed the command tail and head as it is now better documented in the man files and to avoid duplicate.

[mdaadoun]

As for the Manual.txt, I removed the commands head/tail.

But maybe we could keep the list of all programs with a short description.

Separated by built-in/external.

Just for ref and keep usage for the man files.

[bill88t]

The Manual.txt is meant to a complete manual, containing everything you could possbly need. There is no need to keep it short.