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
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48225