pgsql: Define PG_TBLSPC_DIR for path pg_tblspc/ in data folder

Define PG_TBLSPC_DIR for path pg_tblspc/ in data folder

Similarly to 2065ddf5e34c, this introduces a define for "pg_tblspc".
This makes the style more consistent with the existing PG_STAT_TMP_DIR,
for example.

There is a difference with the other cases with the introduction of
PG_TBLSPC_DIR_SLASH, required in two places for recovery and backups.

Author: Bertrand Drouvot
Reviewed-by: Ashutosh Bapat, Álvaro Herrera, Yugo Nagata, Michael
Paquier
Discussion: https://postgr.es/m/ZryVvjqS9SnV1GPP@ip-10-97-1-34.eu-west-3.compute.internal

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c7cd2d6ed082a4638172acece33ed6f36da96263

Modified Files
--------------
src/backend/access/transam/xlog.c           | 12 ++++++------
src/backend/access/transam/xlogrecovery.c   | 15 ++++++++-------
src/backend/backup/backup_manifest.c        |  3 ++-
src/backend/backup/basebackup.c             |  2 +-
src/backend/commands/dbcommands.c           |  2 +-
src/backend/commands/tablespace.c           |  4 ++--
src/backend/storage/file/fd.c               | 29 +++++++++++++++--------------
src/backend/storage/file/reinit.c           | 10 +++++-----
src/backend/utils/adt/dbsize.c              |  8 ++++----
src/backend/utils/adt/misc.c                |  4 ++--
src/backend/utils/cache/relcache.c          |  4 ++--
src/bin/pg_checksums/pg_checksums.c         |  6 +++---
src/bin/pg_combinebackup/pg_combinebackup.c | 18 +++++++++---------
src/bin/pg_rewind/file_ops.c                |  2 +-
src/bin/pg_upgrade/exec.c                   |  2 +-
src/common/file_utils.c                     |  3 ++-
src/common/relpath.c                        | 25 +++++++++++++++----------
src/include/common/relpath.h                |  9 +++++++++
18 files changed, 88 insertions(+), 70 deletions(-)