[krbdev.mit.edu #3964] "kdb5_util load" to existing db doesn't work, needed for kpropd

[Ken Raeburn via RT]

Database propagation to a slave KDC depends on being able to run "kdb5_util load" and have it 
replace the existing database after a successful load.  This doesn't work since the DAL 
integration.  (See also lack of sufficient test cases.)

Proposed fix for 1.5: New argument to create/open saying "this is a temporary database, not 
the live one", and a new operation "promote_db" to make the temporary db become the live one.  
This functionality won't work for ldap; deal with that later.