timeout(1): Add -v/--verbose option to show diagnosis info

The -v/--verbose option enables this utility to show diagnosis
info to stderr about any signal sent on timeout.

This implementation refers to GNU coreutils's timeout(1).

Reviewed by: bapt, Alexander Ziaee (manpages)
Approved by: bapt (src)
Obtained from: DragonFlyBSD
Differential Revision: https://reviews.freebsd.org/D48225

(cherry picked from commit d633a7d12105a54551622882f4eee80a13a1445a)