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.

Behavior change: previous code didn't copy PROXY from the
header_ticket, but this seems to have been a minor bug rather than
intentional.  This also seems to have been an omission from RFC 4120.
Remove compile_et.c

We have used awk scripts in preference to the C version of compile_et
since 1994 (r4426).  Get rid of the sources for the C version and all
of the Makefile.in references to them.