Tweak planner to use a minimum size estimate of 10 pages for a

Tweak planner to use a minimum size estimate of 10 pages for a
never-yet-vacuumed relation.  This restores the pre-8.0 behavior of
avoiding seqscans during initial data loading, while still allowing
reasonable optimization after a table has been vacuumed.  Several
regression test cases revert to 7.4-like behavior, which is probably
a good sign.  Per gripes from Keith Browne and others.