Fix pg_dumpall so that when --clean is specified, it drops roles and

Fix pg_dumpall so that when --clean is specified, it drops roles and
tablespaces in an order that has some chance of working.
Per a complaint from Kevin Bailey.

This is a pre-existing bug, but given the lack of prior complaints I'm
not sure it's worth back-patching.  In most cases failure of the DROP
commands wouldn't be that important anyway.

In passing, fix syntax errors in dumpCreateDB()'s queries for old servers;
these were apparently introduced in recent binary_upgrade patch.