This tries to add information about when each option was added. It goes
back to version 183.
The version info is included from a separate file to allow generating it,
which would allow more control on the formatting of the final output.
Systemd documents "halt" as the primary shutdown mechanism, redirecting
"reboot" and "shutdown" to the halt(8), but halt is a really strange and
obsolete concept. Who would want to really keep their machine running after
shutdown? I expect that halting is almost unused. Let's at least make it less
prominent in the docs.
While at it, use "power off" for a verb and "power-off" for noun (but "poweroff"
of the actual command name).
