runtime.sgml said that you couldn't change SysV IPC parameters on OpenBSD
except by rebuilding the kernel.  That's definitely wrong in OpenBSD 6.x,
and excavation in their man pages says it changed in OpenBSD 3.3.

Update NetBSD and OpenBSD sections to recommend adjustment of the SEMMNI
and SEMMNS settings, which are painfully small by default on those
platforms.  (The discussion thread contemplated recommending that
people select POSIX semaphores instead, but the performance consequences
of that aren't really clear, so I'll refrain.)

Remove pointless discussion of SEMMNU and SEMMAP from the FreeBSD
section.  Minor other wordsmithing.

Discussion: https://postgr.es/m/27582.1546928073@sss.pgh.pa.us