Give up on running with NetBSD/OpenBSD's default semaphore settings.

Give up on running with NetBSD/OpenBSD's default semaphore settings.

This reverts commit 38da053463bef32adf563ddee5277d16d2b6c5af, which
attempted to preserve our ability to start with only 60 semaphores.

Subsequent changes (particularly 55b454d0e) have put that idea pretty
much permanently out of reach: people wishing to use Postgres v18 on
OpenBSD or NetBSD will have no choice but to increase those platforms'
default values of SEMMNI and SEMMNS.

Hence, revert 38da05346's changes in SEMAS_PER_SET and the minimum
tested value of max_connections.  Adjust a comment from the subsequent
patch 6d0154196, and tweak the wording in runtime.sgml to make it
clear that changing SEMMNI/SEMMNS is no longer even a little bit
optional on these platforms.

Although 38da05346 was later back-patched into v17, leave that branch
alone: it's still capable of starting with 60 semaphores, and there's
no reason to break that.

Author: Tom Lane <[email protected]>
Reviewed-by: Nathan Bossart <[email protected]>
Discussion: https://postgr.es/m/[email protected]
Discussion: https://postgr.es/m/1052019.1745947915@sss.pgh.pa.us