Several Improvements - Making the script easier and more comfortable to use :)

Improvements

Added DBG mode (turned on via etc/ykfde.conf if things don't work like they should and you would like to exactly understand what is going on ;))
Added error codes + messages and added few more sanity checks (hook/ykfde)
Added Documentation (to ykfde.conf and to hook/ykfde)
Added Parameters (see ykfde.conf - e.g. slot, parameterized the sleep 5, because I don't need it ... ;)
Added Possibility to combine Password with Challenge-Response
Made the hook/ykfde script overall more robust against typos, less error prone
Added YubiKey detection (to complete the wait for yubiKey functionality of hook/ykfde script)
Added a testrun.sh Test script to test the hook not first during boot-up ;)