openrc-init: include sysvinit wrappers

https://github.com/OpenRC/openrc/blob/08d7def0fd26810e135485aab0397f257bd3f944/init-guide.md#optional-sysvinit-compatibility says

If you build and install OpenRC with MKSYSVINIT=yes, you will build and install wrappers that make openrc-init compatible with sysvinit -- you will have commands like "halt" "shutdown" "reboot" and "poweroff".

Indeed, trying to use busybox reboot on a openrc-init system doesn't work, which is pretty confusing. It would be nice if the openrc-init installed these compatibility shims.