Use a single global dump for iprop full syncs

Use a global dump (the default dump file) for full syncs for iprop.
When a slave asks for a fullsync we kprop the existing global dump to it
if that is good enough, else we dump the DB and send the new global
dump.

Before this change kadmind would run kdb5_util dump -i... each time a
slave asked for a full dump.  This was done in a sub-process,
thankfully, but it was still a waste of time and storage (e.g., if one
has a huge KDB).

Also, long dump times might cause a slave to give up.  But since iprop
dumps bear a serial number and timestamp and since slaves will resync
from that point forward, it doesn't matter if the dump we send a slave
is fresh as long as it is fresh enough (i.e., that its sno and timestamp
are in the ulog).

Also:

 - Rename dumps into place instead of unlink, create, write (but we
   still keep the dump ok files for backwards compatibility).

 - Refuse to accept non-iprop dumps when loading if iprop is enabled and
   the ulog has seen at least one entry.

   NOTE: This may necessitate a change in default dump format on the
         master side so that dumps use ipropx format by default.

 - Fix a race condition relative to the kadmind side's starting of
   the full sync's kprop (which might otherwise trigger often now that
   kadmind does not dump the KDB every time a slave asks for a full
   resync).

   Make kpropd in iprop mode fork a child to listen for kprops from the
   master.  The child writes progress and outcome reports to the parent
   for each kprop.

 - Add iprop timeout params for full resyncs: one for timing out the
   start of the requested (and granted) full resync (this corresponds to
   dump times on the master), and one for timing out hung transfers
   after a transfer has started.  We do not have a timeout on the
   kdb5_util load operation.

 - Add runonce (-t) for kpropd do_standalone() too.

 - Add -p and -F CLI options to kadmind like kpropd's so that we can
   use alternate kdb5_util commands and alternate dump locations.
   This is necessary for testing iprop.

 - Add a -C kprop-path option to kadmind so we can tell it which kprop
   to use for full resyncs.  This is needed, e.g., for the iprop test.

 - Make kpropd log stats for iprop.  This is useful for sizing ulogs.
Update kpropd.rst
Deprecate kpropd -S option
kadmind iprop debugs messages should go to stderr
kdb5_util load needs iprop safety net
Add -R option to kproplog to force full resyncs
Prep k5test.py for iprop (add start_kpropd(), ...)

Add a start_kpropd() method to K5Realm and make start_kadmind() use the
kadmind -p, -C, and -F options.
Import Greg Hudson's iprop test and get it closer
Stop returning UPDATE_BUSY

Currently kadmind allows slaves to poll for updates as often as they
like, but not within 10s of the last update.  This means that iprop will
appear to fail to synchronize the KDC at any site whose master KDC
processes at least one write transaction every 10 seconds consistently.

The original intention must have been to throttle iprop clients (slave
KDCs) that poll too often.  But UPDATE_BUSY as implemented is not that.
Remove "kpropd: " from logs; always openlog()
Cleanup some debug/syslog msgs and C style
Stop localizing log msgs; debug msgs to stderr