Allow extension tables to be dumped.

Not dumping first five rows in pga_jobclass, though, as they are inserted when
the extension is created. Added a comment that the number should be updated if
new classes are added -- although that will make dump, upgrade, and restore a
PITA. Long term, there might need to be an additional column in pga_jobclass to
mark pre-defined classes.

Or, if users cannot add new classes, then we can remove the code to dump it
altogether.