Add tests for S4U request-authdata handling

Add tests for S4U request-authdata handling

In adata.c, look up the server in the keytab by ticket->server (which
has the canonicalized realm), to allow testing of cross-realm RBCD
(although unused for now).

In s4u2proxy.c, set KRB5_GC_CANONICALIZE to support RBCD, and add an
authdata request option.  Add an s4u2self test harness with authdata
request option.

[ghudson@mit.edu: minor code simplifications; edited commit message]

ticket: 8866