Make REINDEX DATABASE do what one would expect, namely reindex all indexes

Make REINDEX DATABASE do what one would expect, namely reindex all indexes
in the database.  The old behavior (reindex system catalogs only) is now
available as REINDEX SYSTEM.  I did not add the complementary REINDEX USER
case since there did not seem to be consensus for this, but it would be
trivial to add later.  Per recent discussions.