OpenRC init scripts for setting up dm-cache on Gentoo Linux

Full instructions at https://wiki.tnonline.net/w/Linux/dm-cache

Setup:
* Copy 'dmcache.initd' to '/etc/init.d/dmcache_my_cache'
* Copy 'dmcache.confd' to '/etc/conf.d/dmcache_my_cache'
* Copy '90-dmcache.rules' to '/etc/udev/rules.d/'
* Make sure '/etc/conf.d/dmcache_my_cache' is executable.
* Modify '/etc/conf.d/dmcache_my_cache' and 
  '/etc/udev/rules.d/90-dmcache.rules' with appropiate devices.
* for autostart add dmcache to the 'boot' runlevel
  `rc-update add dmcache_my_cache boot`

If you have several devices you simply make a copy of the
init.d and config.d files to a new name. The filenames in
init.d and conf.d must be the same.

# SPDX-License-Identifier: GPL-3.0-or-later
# Copyright 2023 Forza <forza@tnonline.net>
# https://git.tnonline.net/Forza/dm-cache