pg_dump, pg_upgrade: allow postgres/template1 tablespace moves

Modify pg_dump to restore postgres/template1 databases to non-default
tablespaces by switching out of the database to be moved, then switching
back.

Also, to fix potentially cases where the old/new tablespaces might not
match, fix pg_upgrade to process new/old tablespaces separately in all
cases.

Report by Marti Raudsepp

Patch by Marti Raudsepp, me

Backpatch through 9.0