libthr: thr_attr.c: Clarity, whitespace and style

Also, remove most comments, which don't add value.

Reviewed by: emaste
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43005

(cherry picked from commit bd61c1e89dc4a40ba696de1785d423978e1c2147)

Approved by: markj (mentor)