Assign a status string if tkt_id generation fails

Jumping to egress with no status will cause the KDC to abort.
Fix memory leak
Do not send stack garbage to plugins for early errors
Do not tell lies about JSON encoding routines
Replace dead code with a reachable check
Spell NULL correctly

This function takes a pointer argument, not an integer.
Rewrite kau_init_kdc_req output handling to be style-compliant
Spell kau_as_req and kau_tgs_req more conventionally
Remove duplicate assignment
Leave a breadcrumb for preauth type additions
Use --enable-audit-plugin for the configure arg

Also put a more helpful comment in the m4 code.
Tweak some comments