Allow "in place" tablespaces.

Allow "in place" tablespaces.

Provide a developer-only GUC allow_in_place_tablespaces, disabled by
default.  When enabled, tablespaces can be created with an empty
LOCATION string, meaning that they should be created as a directory
directly beneath pg_tblspc.  This can be used for new testing scenarios,
in a follow-up patch.  Not intended for end-user usage, since it might
confuse backup tools that expect symlinks.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CA%2BhUKGKpRWQ9SxdxxDmTBCJoR0YnFpMBe7kyzY8SUQk%2BHeskxg%40mail.gmail.com