vfprintf(3): Add support for kernel %b format

This is a direct port of the kernel %b format.

I'm unclear on if (more) non-portable printf extensions will be a
problem. I think it's desirable to have userspace formats include all
kernel formats, but there may be competing goals I'm not aware of.

Reviewed by: no one, unfortunately
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8426