Suppress extensions in partial dumps.

Suppress extensions in partial dumps.

We initially had pg_dump emit CREATE EXTENSION commands unconditionally.
However, pg_dump has long been in the habit of not dumping procedural
language definitions when a --schema or --table switch is given.  It seems
appropriate to handle extensions the same way, since like PLs they are SQL
objects that are not in any particular schema.  Per complaint from Adrian
Schreyer.