newvers: Add comment about why we need sccs, but deprecate it

The SCCS ID is still the most reliable way to dig out the version
information from the kernel w/o false positives. Add a comment to that
effect. savecore(8) neglects to save the kerneldumpheader that would
have the version information at a fixed location. savecore(8) needs to
be augmented to have the right data in the right places, but until then
the old-school SCCS id needs to remain. Once that's fixed, we plan to
remove it.

The reason it needs to be in a fixed or easily findable location is
because if you have an arbitrary core and want to pull the source and
build artificts that went along with that core, you don't yet have the
symbols you need to read the version string. To solve the chicken / egg
problem, one needs an independent way to know what to use so that
automated analysis of cores can happen. The sccs id being in the kernel
ensures that it is in the core image written. The what(1) utility makes
extracting the version easy.

Sponsored by: Netflix
Reviewed by: cy, emaste
Differential Revision: https://reviews.freebsd.org/D41984