Make init scripts work properly for Debian+OpenRC setup (again)

[heroxbd]

Motivation and Context

I intend to submit a sequel of #8359 to finally produce the correct init scripts for Debian + OpenRC setup.

Let Debian use the sysv-rc variant of the script, even when OpenRC is installed. Unlike on Gentoo, OpenRC on Debian consumes both the sysv-rc scripts and OpenRC ones. ZFS initscripts on Debian should be the sysv-rc version to provide most compatibility and to integrate with the rest of initscripts for dependency tracking.

Past issues closed by being staled includes:
#8063
#8204

Description

It replaces the runtime check with build-time configuration, simplifying the logic to maintain initscripts of ZFS. The built init script is completely determined by the --with-vendor= option.

This construct is inspired by Mo Zhou's runtime detection of the execution shell.

How Has This Been Tested?

Debian unstable + sysv-rc: works
Debian unstable + openrc: works
Gentoo + openrc: works

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes. (not needed)
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[heroxbd]

Hi Mo (@cdluminate), you comments would be high valuable to this PR.
Hi Arvind @nivedita76, you have authored the last factorization of the Makefile substitutions. I am moving the initscript substitution logic back to autoconf. Looking forward to your input.

[cdluminate]

My memory about the Debian OpenRC + ZFS setup is almost gone. But after going through the patch, I think there is a clear problem. The IS_SYSV_RC replacement variable is evaluated during the automake process and is permanently fixed afterwards.

Think about the following scenario: (1) the user installs Debian/gentoo with sysv-rc; (2) the user installs openzfs and the generated scripts are specific to sysv-rc; (3) the user decides to switch to openrc and zfs services just breaks silently.

Is it possible to detect this within the service scripts at the run time?

[heroxbd]

My memory about the Debian OpenRC + ZFS setup is almost gone. But after going through the patch, I think there is a clear problem. The IS_SYSV_RC replacement variable is evaluated during the automake process and is permanently fixed afterwards.

Think about the following scenario: (1) the user installs Debian/gentoo with sysv-rc; (2) the user installs openzfs and the generated scripts are specific to sysv-rc; (3) the user decides to switch to openrc and zfs services just breaks silently.

Is it possible to detect this within the service scripts at the run time?

This is not a concern, because OpenRC on debian consumes both sysv-rc and openrc initscripts. (3) won't happen.