Fix pg_dumpall with in-place tablespaces

Fix pg_dumpall with in-place tablespaces

In-place tablespaces would be dumped with the path produced by
pg_tablespace_location(), which is in this case a relative path built as
pg_tblspc/OID, but this would fail to restore as such tablespaces need
to use an empty string as location.  In order to detect if an in-place
tablespace is used, this commit checks if the path returned is relative
and adapts the dump contents in consequence.

Like the other changes related to in-place tablespaces, no backpatch is
done as these are only intended for development purposes.  Rui Zhao has
fixed the code, while the test is from me.

Author: Rui Zhao, Michael Paquier
Discussion: https://postgr.es/m/80c80b4a-b87b-456f-bd46-1ae326601d79.xiyuan.zr@alibaba-inc.com