newvers: append commit count to uname version string

In a git world this provides a facsimile of a monotonically increasing
version number. This might be refined further, but this provides a
starting point for investigation.

Reviewed by: cem
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20462

The "Relnotes" flag in the commit message is removed in cherry-pick as
git repo doesn't affect the stable/11 and stable/12 release.

(cherry picked from commit ecb2bbc081ae906956f4718ec946b9e4fcea241f)