Add __diagused

This can be used for variables which are only used with either
INVARIANTS or WITNESS. Without any annotation they run into dead store
warnings from cc --analyze and always annotating with __unused may hide
bad vars when it should not.

Reviewed by: kib, markj
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31185

(cherry picked from commit b9296725cf99696b48926270a553044522d6e344)