Document kpropd -t and fix it in iprop mode

If kpropd is asked to run just once, don't exit after starting a full
resync; we want to wait for the fullprop child to process the request,
and then request incremental updates afterwards.  Also don't exit from
do_standalone() in the fullprop child, in case multiple full resyncs
are required to get the database up to date.

Document the -t flag in kpropd.rst.

https://github.com/krb5/krb5/commit/334f761ba485e4af6d5fb8822e276d4590b97bf5
Author: Greg Hudson <ghudson@mit.edu>
Commit: 334f761ba485e4af6d5fb8822e276d4590b97bf5
Branch: master
 doc/admin/admin_commands/kpropd.rst |    7 +++++++
 src/slave/kpropd.c                  |    4 ++--
 2 files changed, 9 insertions(+), 2 deletions(-)