Since we already generate the man pages automatically,
we can generate the bash completions too.
It is much faster and does not risk making any network requests.

fixes #194

For system installation, the bash auto-completion will display messages
for users without home directory. Redirect stderr to null

* Add files to the possibilities (needed at least for pastoob).
  For now show them only if nothing else  matches.
* Remove useless "$have" test that failed every time
* Forget fake "hash" after running
* Simpler code overall