Simplify get_in_tkt.c restart handling

Simplify get_in_tkt.c restart handling

To simplify callers, make restart_init_creds_loop() reset the
err_reply and err_padata fields and free per-request preauth moddata.
Change its padata argument to a boolean argument for FAST upgrades,
instead of sometimes passing in ctx->err_padata (which would become
invalid partway through the function now that we're freeing it).
Split up the upgrade-to-FAST and downgrade-to-no-padata cases in
init_creds_step_reply(), and eliminate negotiation_requests_restart().

For brevity, rename the krb5_init_creds_context have_restarted field
to restarted.  Rename krb5int_upgrade_to_fast_p() to
k5_upgrade_to_fast_p() and make it a true predicate.  Change some flag
field assignments to use TRUE/FALSE instead of 1/0.  Reset
enc_pa_rep_permitted after a client realm referral, since we don't
know that the new realm's KDCs will fail on informational padata.