Multiple pid-1 / service managers

This issue is to coordinate the addition of dinit and s6 support in alpine linux (and maybe more). Edit: This issue is to coordinate changes to the way pid-1 and service managers are handled/packaged in alpine linux.

Note the important distinction between pid-1 and service manager! They can be both the same binary though (eg. dinit).

• pid-1: busybox init, openrc-init, s6-linux-init, runit-init, dinit-init
• service manager: openrc, s6-rc, runit, dinit, busybox runit

This is what I came up with as important steps (order irrelevant):

• Remove busybox init bias (Draft !59848)
  • Add busybox-init subpackage depending on /etc/inittab
  • Split /etc/inittab from alpine-baselayout into openrc-inittab subpackage providing /etc/inittab
• Create meta packages that are used in the install_if of service packages. (eg. install_if=dinit-system-services)
  • openrc-system-services
  • dinit-system-services
• pid-1 provide /sbin/init
• Create a service format that can generate OpenRC, dinit and s6 service files (for reducing effort) (Testing cross-services)