JSON: Implemented last data update key in results object.

The last_data_update key in the results object has been added to reflect
how current the data in the data object might be.

- For object JSON queries (except help), the last data update is the
  modified time on the objects.cache file.
- For status JSON queries (except help), the last data update is the
  modified time on the status.dat file.
- For archive JSON queries (except help), the last data update is the
  modified time on most recent archive log file read.
- For the help query in any JSON CGI, the last data update is the time
  the source code was compiled.
- In the event of an error, the last data update will be one second
  before the beginning of the Unix epoch, Dec 31, 1969 at 23:59:59 UTC.
Archive JSON: Last data update now set on warnings

This commit adds code to fetch a member of the JSON structure based on
a path to the member. In the event that a warning condition is
encountered because of passed parameters, this code is used to fetch
the result object so the last data update key can be updated.

This commit also initializes the value of the last data update so we
don't end up with last data update times thousands of years in the
future.