Add an internal-use setter accessor for use by mssfu.c when principal names are
replaced. This also fixes a leak where r->client_princ was not freed before
being replaced with the impersonated client name.
Remove hdb_entry_ex and revert to the original design of hdb_entry (except with
an additional context member in hdb_entry which is managed by the free_entry
method in HDB).
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.
Move the Services for User (SFU/S4U) implementation -- protocol transition and
constrained delegation -- into its own compilation unit, with an interface that
only takes an astgs_request_t, so it can be easily factored out into a plugin
module in the future.
This refactoring is also careful to update all client names in the request
structure after the SFU/S4U validation has successfully completed.
