Teach pg_upgrade to handle in-place tablespaces.

Presently, pg_upgrade assumes that all non-default tablespaces
don't move to different directories during upgrade.  Unfortunately,
this isn't true for in-place tablespaces, which move to the new
cluster's pg_tblspc directory.  This commit teaches pg_upgrade to
handle in-place tablespaces by retrieving the tablespace
directories for both the old and new clusters.  In turn, we can
relax the prohibition on non-default tablespaces for same-version
upgrades, i.e., if all non-default tablespaces are in-place,
pg_upgrade may proceed.

This change is primarily intended to enable additional pg_upgrade
testing with non-default tablespaces, as is done in
006_transfer_modes.pl.

Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aA_uBLYMUs5D66Nb%40nathan

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