Could also move kdc.conf, but this makes it inconvenient to use command line
utilities with heimdal, as it would require specifying --config-file with every
command.
Allow switching out kerberos server implementation.
Sharing config is probably sensible, but implementation is different enough to
be worth splitting into two files. Not sure this is the correct way to split an
implementation, but it works for now.
Uses the switch from config.krb5 to select implementation.
