expand_number: Tighten check of unit.

The current code silently ignores characters after the unit as long
the unit themselves were recognized. This commit makes expand_number(3)
to fail with EINVAL if buf did not terminate after the unit character.

Historically, the function accepts and ignores "B" as a SI unit, this
behavior is preserved and e.g. KB, MB are still accepted as aliases of
K and M, document this behavior in the manual page.

While I am there, also write a few test cases to validate the behavior.

Reviewed-by: emaste
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D40482