device_id: improve searched_devnames temp file

device_id: improve searched_devnames temp file

Include info in the temp file to confirm that it should be used.
The temp file is meant to suppress repeated, identical searches
for the same PVIDs on the same set of devices.  Write to the file
a count and hash of the missing PVIDs and a count and hash of the
devices to search.  A subsequent command will ignore and remove
the temp file if any of these values differ.  We don't want to
suppress a search if a change has occured, and a missing PV could
be found by scanning devices.