Add more configuration parameters, fix various bugs

Add more configuration parameters, fix various bugs

Add config_file, db_name, and stash_file configuration parameters
and pass them into either libkadm5srv or the Kerberos context as
needed.

Fix the code to set the elements of the exception thrown from the
Kerberos kadmin code.  hv_stores is a better API to use here.

Use KADM5_ADMIN_SERVICE instead of a hard-coded string.

Include the Authen::Kerberos::Exception class in the
Authen::Kerberos::Kadmin class so that exceptions will stringify
and compare properly by default.