Boolify should_yield()

Do this ahead of adding a man page that describes the function. No
functional change.

Reviewed by: kib, markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38181

(cherry picked from commit d570418bd8918980e578ff78a229593766b832bb)