pgsql: Prohibit max_slot_wal_keep_size to value other than -1 during up

Prohibit max_slot_wal_keep_size to value other than -1 during upgrade.

We don't want existing slots in the old cluster to get invalidated during
the upgrade. During an upgrade, we set this variable to -1 via the command
line in an attempt to prevent such invalidations, but users have ways to
override it. This patch ensures the value is not overridden by the user.

Author: Kyotaro Horiguchi
Reviewed-by: Peter Smith, Alvaro Herrera
Discussion: http://postgr.es/m/20231027.115759.2206827438943188717.horikyota.ntt@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8bfb231b43d7f6058041483f2b823dd52eac7bf8

Modified Files
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src/backend/access/transam/xlog.c   | 19 +++++++++++++++++++
src/backend/replication/slot.c      | 16 +++++-----------
src/backend/utils/misc/guc_tables.c |  2 +-
src/include/utils/guc_hooks.h       |  2 ++
4 files changed, 27 insertions(+), 12 deletions(-)