timeout(1): Some minor tweaks and improvements

• Define exit status and macros and use them
• Improve the second kill logic by setting 'do_second_kill = false' after configuring the second kill
• Minor style tweaks
• Reorder options in the man page, as well as the usage help
• Reorder the exit status in the man page
• Enhance the HISTORY section in the man page (obtained from NetBSD)

Reviewed by: bapt, Alexander Ziaee (manpages)
Approved by: bapt (src)
Obtained from: DragonFlyBSD
Differential Revision: https://reviews.freebsd.org/D47866

(cherry picked from commit 06690044dac183ea1d93c2ae227e261da3bdca2a)