Add a helper class that provides readline-based tab completion and an
extra optional argument "completion_choices" with possible completion
targets.
While at this, also improve paramters documentation for ask() func.
While at it, also remove unnecessary "#!/usr/bin/env python3" in files
that only get imported, and adjust other empty/comment lines in the
beginnings of the files for consistency.
This makes files easier to read, and makes the pmbootstrap codebase more
consistent with the build.postmarketos.org codebase.
* Validate all inputs from `pmbootstrap init`
* Add a new `confirm()` function, that validates input of yes/no
questions properly
* All questions loop until they have a valid answer now
