Log service princ in KDC more reliably

Under some error conditions, the KDC would log "<unknown server>" for
the service principal because service principal information is not yet
available to the logging functions.  Set the appropriate variables
earlier.

do_as_req.c: After parsing the client, immediately unparse the server
before searching for the client princ in KDB.

do_tgs_req.c: Set the service principal to the request-specified
principal if search_sprinc() returns an error, to ensure that it gets
logged.

[tlyu@mit.edu: commit message; minor code style changes]