pgsql: Define PG_REPLSLOT_DIR for path pg_replslot/ in data folder

Define PG_REPLSLOT_DIR for path pg_replslot/ in data folder

This commit replaces most of the hardcoded values of "pg_replslot" by a
new PG_REPLSLOT_DIR #define.  This makes the style more consistent with
the existing PG_STAT_TMP_DIR, for example.  More places will follow a
similar change.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2065ddf5e34ce098f549c4279ee3ab33c188a764

Modified Files
--------------
src/backend/backup/basebackup.c                 |  3 +-
src/backend/replication/logical/reorderbuffer.c | 17 ++++-----
src/backend/replication/slot.c                  | 46 ++++++++++++-------------
src/backend/utils/adt/genfile.c                 |  5 +--
src/bin/pg_rewind/filemap.c                     |  2 +-
src/include/replication/slot.h                  |  3 ++
6 files changed, 41 insertions(+), 35 deletions(-)