Expand dns_canonicalize_host=fallback support

Expand dns_canonicalize_host=fallback support

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
k5_get_proxy_cred_from_kdc().

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.

Add fallback processing to krb5_rd_req().

ticket: 8930 (new)