Remove the concept of filespaces, revert tablespaces to work the same as
in upstream.

There is some leftovers in management tools. I don't know how to test all
that, and I was afraid of touching things I can't run. Also, we may need
to create replacements for some of those things on top of tablespaces, to
make the management of tablespaces easier, and it might be easier to modify
the existing tools than write them from scratch. (Yeah, you could always
look at the git history, but still.)

Per the discussion on gpdb-dev mailing list, the plan is to cherry-pick
commit 16d8e594 from PostgreSQL 9.2, to make it possible to have a
different path for a tablespace in the primary and its mirror. But that's
not included in this commit yet.

TODO: Make temp_tablespaces work.
TODO: Make pg_dump do something sensible, when dumping from a GPDB 5 cluster
that uses filespaces. Same with pg_upgrade.

Discussion: https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/sON4lraPEqg/v3lkM587BAAJ