hdb: decorate HDB_entry with context member

Decorate HDB_entry with context and move free_entry callback into HDB structure
itself. Requires updating hdb_free_entry() signature to include HDB parameter.
A follow-up commit will consolidate hdb_entry_ex (which has a single hdb_entry
member) into hdb_entry.

kdc/kdc.h

@@ -148,10 +148,12 @@ typedef struct krb5_kdc_configuration {
     /* server principal */					\
     krb5_principal server_princ;				\
     hdb_entry_ex *server;					\
+    HDB *serverdb;						\
 								\
     /* presented ticket in TGS-REQ (unused by AS) */		\
     krb5_principal *krbtgt_princ;				\
     hdb_entry_ex *krbtgt;					\
+    HDB *krbtgtdb;						\
     krb5_ticket *ticket;					\
 								\
     krb5_keyblock reply_key;					\