Regression tests for keyless principals

Confirm that kadmind does not crash when creating/modifying a principal
to have no keys, and confirm that no keys are present after a
purgekeys -all.
Remove some dead code

The secretkey variable is initialized to NULL and compared against
NULL, but never actually set to anything after initialization.

Remove the variable and all code that would have executed if it
was non-NULL.
Add helper for freeing arrays of berval pointers

This eliminates a potential leak of the bv_val members from
krb5_encode_krbsecretkey().
Correct spelling

Remove extra 'i' from "create_standalone_prinicipal".