ktrace(1): add more xrefs

Following commit a87651e2ff18 add xrefs to intro(2) and sigaction(2),
and use a consistent form.

Suggested by: kib, arrowd
Reviewed by: kib (earlier)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45555

(cherry picked from commit 0277c0c6f72a964bf0439db1f4558551052e4f3b)