Use stderr and return codes >0 when it makes sense.
Loose return code conventions:
* 1 for generic errors
* 2 for user input errors
* 3 for remote errors (not found, etc.)
* 4 for not implemented

display usage, description and commands list in a proper way, display copyright and files list (refs #427)

Far from enough to replace the current manpages as of now, so they are
put in the man2/ directory.