Remove restart(); add framework for reload() support

[skarnet]

OpenRC will never call restart(), so scrap the definition in
openrc-run.sh and s6.sh.
reload() caused a weird error message when used without
supervisor=s6, so add proper support for generic reload(), instantiated for s6, and with a better default error message for other backends - which can now add support for reload() down the line.

Also clarify a comment, and add info messages on s6_reload.

[navi-desu]

i think i'd rather print an error like reload not defined (seems like we currently print $command: not found, i'll improve that latter), saying "only supported with supervisor=s6" sounds like it can't be used with anything else -- though i can fix it myself while merging if you'd like

[skarnet]

That's why I wrote "currently", I think? But yeah, please merge and change the ewarn message to whatever feels better to you, the message is just a placeholder.