style(9): Default to omitting $FreeBSD$

Advise people to omit $FreeBSD$ (in both comments and macros) unless the
code is definitely going to be merged to stable/12. This strengthens
previous statements and is appropriate now that stable/11 is no longer
supported. If people are wrong and things are unexpected merged to 12,
tags can be added before that merge. No sense adding a tag that will
never be expanded and removed later on the off chance it might wind up
in stable/12.

The next step is likely to weaken this to apply just to mergemaster
managed files, but not today.

Reviewed by: rpokala, cem, erj, hselasky, brooks, kib
Differential Revision: https://reviews.freebsd.org/D34152