Tables: Allow sorting inactive time stamps.

Tables: Allow sorting inactive time stamps.

Hsui-Khuern Tang writes:

   I have a table where one column consists of inactive dates, such as
   this:

     | Date             | OK? |
     |------------------+-----|
     | [2009-01-30 Fri] | x   |
     | [2009-01-27 Tue] | x   |
     | [2009-01-28 Wed] | x   |

   I can't sort the table by that column.  If I change the dates to
   active, then sorting (by time) works fine.  Is this behavior
   intentional?

Yes, intentional, because the same routine also does sorting of
entries and should prefer active time stamps.  However, I realize that
we can still look for an inactive time stamp when no active one can be
found.   This is what this commit implements.