Improve KDC encrypting key memory management

Improve KDC encrypting key memory management

Commit 0ba5ccd7bb3ea15e44a87f84ca6feed8890f657d caused encrypting_key
to hold either an owned or alias pointer depending on the presence of
the KDC_OPT_ENC_TKT_IN_SKEY (u2u) flag in the request.  For better
memory management safety, introducing server_keyblock to hold owned
memory in the non-u2u case, and turning encrypting_key into an alias
pointer for both cases.