This replaces the krb5 module's options with RFC 42-style krb5.settings
option, while greatly simplifying the code and fixing a few bugs,
namely:
- #243068 krb5: Configuration silently gets ignored when set by
multiple modules
- not being able to use mkIf etc. inside subattributes of
krb5.libdefaults, e.g. krb5.libdefaults.default_realm = mkIf ...
See #144575.
Closes#243068.
Co-authored-by: h7x4 <h7x4@nani.wtf>
