pgsql: Unify src/common/'s definitions of MaxAllocSize.

Unify src/common/'s definitions of MaxAllocSize.

Define MaxAllocSize in src/include/common/fe_memutils.h rather
than having several copies of it in different src/common/*.c files.
This also provides an opportunity to document it better.

Back-patch of commit 11b7de4a7, needed now because assorted security
fixes are adding additional references to MaxAllocSize in frontend
code.

Backpatch-through: 14-17
Security: CVE-2026-6473

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d106295b60578b31da37b7ae95c694e14774411b
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>

Modified Files
--------------
src/common/psprintf.c | 3 ---
src/common/saslprep.c | 4 ----
src/common/stringinfo.c | 3 ---
src/include/common/fe_memutils.h | 12 ++++++++++++
4 files changed, 12 insertions(+), 10 deletions(-)