Add PrincipalRef to deal with aliases

Subclass Principal into PrincipalRef to deal with principals that must
not be directly freed by garbage collection, e.g., because they're
aliases into structures such as krb5_enc_tkt_part.

lib/krb5.py

@@ -153,6 +153,15 @@ def __str__(self):
     def __repr__(self):
         return '<%s: %s>' % (self.__class__.__name__, self.unparse_name())
 
+# A reference to a principal, to avoid freeing
+class PrincipalRef(Principal):
+    def __init__(self, ctx, princ):
+        self._ctx = ctx
+        self._handle = princ
+
+    def __del__(self):
+        pass
+
 class Credentials(object):
     def __init__(self, ctx):
         self._ctx = ctx