Expose the kernel's build-ID through uname -b

After r348611 the kernel's build-ID is available via sysctl. Add a -b flag
to uname to report it.

Submitted by: Ali Mashtizadeh <ali_mashtizadeh.com>
Reviewed by: markj, imp
Relnotes: Yes
Event: Waterloo Hackathon 2019
Differential Revision: https://reviews.freebsd.org/D20511