In krb5_sname_to_principal(), when using fallback, defer realm lookup
and any kind of hostname canonicalization until use. Add a
lightweight iterator k5_canonprinc() to yield the one or two possible
candidates for a principal. In the iterator, don't yield the same
hostname part twice.
Add fallback processing to the stepwise TGS state machine, and remove
it from krb5_get_credentials(). Add fallback processing to
Add fallback processing to krb5_init_creds_set_keytab(), and use the
principal we find in the keytab as the request client principal.
Defer restart_init_creds_loop() to the first step call so that server
principal is built using the correct realm.