DROP OWNED: don't try to drop tablespaces/databases

My "fix" for bugs #7578 and #6116 on DROP OWNED at fe3b5eb08a1 not only
misstated that it applied to REASSIGN OWNED (which it did not affect),
but it also failed to fix the problems fully, because I didn't test the
case of owned shared objects.  Thus I created a new bug, reported by
Thomas Kellerer as #7748, which would cause DROP OWNED to fail with a
not-for-user-consumption error message.  The code would attempt to drop
the database, which not only fails to work because the underlying code
does not support that, but is a pretty dangerous and undesirable thing
to be doing as well.

This patch fixes that bug by having DROP OWNED only attempt to process
shared objects when grants on them are found, ignoring ownership.

Backpatch to 8.3, which is as far as the previous bug was backpatched.
REASSIGN OWNED: handle shared objects, too

Give away ownership of shared objects (databases, tablespaces) along
with local objects, per original code intention.  Try to make the
documentation clearer, too.

Per discussion about DROP OWNED's brokenness, in bug #7748.

This is not backpatched because it'd require some refactoring of the
ALTER/SET OWNER code for databases and tablespaces.