usr.bin/units: use else..if to avoid calling the next branch

Even if the first branch succeeds, next time it will still check for
the second branch (which will be false) as the first one was true.
Add an else..if statement to address this.

Signed-off-by: rilysh <nightquick@proton.me>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1165