Replace remaining parts of FAI

Sadly I think it's time after #53 to remove the remaining parts of FAI. We currently use

• it's ability to call debootstrap,
• it's ability to call scripts at the correct time,
• the package installation stuff and
• the file selection stuff.

Everything else is re-implemented as hook, disabling the original code, and usually implemented in shell.

All of that is kind of clunky and nothing works the same. Error and log handling is broken (hooks can't give errors, the output of scripts is always filtered away and written into files derived from the implementation language). The code is really hard to read.