pgsql: Allow "in place" tablespaces.

Allow "in place" tablespaces.

This is a backpatch to branches 10-14 of the following commits:

7170f2159fb2 Allow "in place" tablespaces.
c6f2f01611d4 Fix pg_basebackup with in-place tablespaces.
f6f0db4d6240 Fix pg_tablespace_location() with in-place tablespaces
7a7cd84893e0 doc: Remove mention to in-place tablespaces for pg_tablespace_location()
5344723755bd Remove unnecessary Windows-specific basebackup code.

In-place tablespaces were introduced as a testing helper mechanism, but
they are going to be used for a bugfix in WAL replay to be backpatched
to all stable branches.

I (Álvaro) had to adjust some code to account for lack of
get_dirent_type() in branches prior to 14.

Author: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Author: Michaël Paquier <michael(at)paquier(dot)xyz>
Author: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Discussion: https://postgr.es/m/20220722081858.omhn2in5zt3g4nek@alvherre.pgsql

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/961cab0a5a90d449a64a71912b324d7e7548619b

Modified Files
--------------
doc/src/sgml/config.sgml | 19 +++++++++++++++++++
src/backend/access/transam/xlog.c | 9 +++++++++
src/backend/commands/tablespace.c | 39 ++++++++++++++++++++++++++++++++-------
src/backend/utils/adt/misc.c | 29 +++++++++++++++++++++++++++++
src/backend/utils/misc/guc.c | 12 ++++++++++++
src/include/commands/tablespace.h | 2 ++
6 files changed, 103 insertions(+), 7 deletions(-)