Mark usage function as __dead2 in programs where it does not return

In most cases, usage does not return, so mark them as dead2. For the
cases where they do return, they have not been marked dead2.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/735

(cherry picked from commit cccdaf507eee8fb34494b4624eb85bb951e323c8)