Add get_principal_keys RPC to kadmin

Add get_principal_keys RPC to kadmin

Change the prototype of kadm5_get_principal_keys() to report kvno and
salt information along with each key.  Add an RPC for extracting keys,
requiring a new permission bit (which is not implied by 'x' or '*' in
kadm5.acl).  Add kadm5_free_kadm5_key_data().

In kadmin, deconditionalize "kadmin ktadd -norandkey".  Use the new
information from kadm5_get_principal_keys() to correctly set the kvno
for each key when existing keys are extracted, fixing issue #7852.

Add tests to t_keytab.py for the #7852 fix.  Add tests to
lib/kadm5/unit-test for the get_principal_keys RPC.

[ghudson@mit.edu: factor out fetch_new_keys() from add_principal();
rewrite commit message to describe new RPC; add #7852 test cases;
squash with lib/kadm5/unit-test commit]

ticket: 8364 (new)