Remove a warning in AES string-to-key

On 32-bit platforms, the code to translate an iteration count of 0 to
2^32 can trigger a compiler warning.  Since we will basically never
accept an iteration count that high (right now we reject anything
above 2^24), just reject it out of hand.
Remove dangling --with-kdc-kdb-update references

This configure option hasn't done anything since 1.8, so don't mention
it in configure --help or the documentation.  The disable_last_success
and disable_lockout DB options are now used to turn it off.
Refactor KDC option/flag processing

A lot of KDC code was spent copying options to flags, and copying
header_ticket flags to the output ticket.

Previous code didn't copy PROXY from the header_ticket, but this seems
to have been a minor bug rather than intentional.