pgsql: Paper over bt_metap() oldest_xact bug in backbranches.

Paper over bt_metap() oldest_xact bug in backbranches.

The data types that contrib/pageinspect's bt_metap() function were
declared to return as OUT arguments were wrong in some cases. In
particular, the oldest_xact column (a TransactionId/xid field) was
declared integer/int4 within the pageinspect extension's sql file. This
led to errors when an oldest_xact value that exceeded 2^31-1 was
encountered.

We cannot fix the declaration on Postgres 11 or 12. All we can do is
ameliorate the problem. Use "%d" instead of "%u" to format the output
of the oldest_xact value. This makes the C code match the declaration,
suppressing unhelpful error messages that might otherwise make
bt_metap() totally unusable. A bogus negative oldest_xact value will be
displayed instead of raising an error.

This commit addresses the same issue as master branch commit 691e8b2e18,
which actually fixed the problem. Backpatch to the 11 and 12 branches
only, since they are the only branches (other than master) that have
oldest_xact. All of the other problematic columns already display bogus
output for out of range values.

Reported-By: Victor Yegorov
Bug: #16285
Discussion: https://postgr.es/m/20200309223557.aip5n6ewln4ixbbi@alap3.anarazel.de
Backpatch: 11 and 12 only

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/393b449f1a643f5a90a4399dffa3148a94e5beec

Modified Files
--------------
contrib/pageinspect/btreefuncs.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)