Clarify lockout replication issues in docs

In the "KDC replication and account lockout" section of lockout.rst,
specifically call out kprop and incremental propagation as the
mechanisms which do not replicate account lockout state, and add a
note that KDCs using LDAP may not be affected by that section's
concerns.

ticket: 7773 (new)
target_version: 1.12
tags: pullup
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.