Set DEFAULT_INIT_SHELL to /sbin/openrc-run for Gentoo and Alpine

[Peter-Levine]

Gentoo and Alpine always set the rc init scripts' shebang to #!/sbin/openrc-run, whether or not openrc is installed.

Motivation and Context

Fixes openzfs#12683

Description

Unconditionally sets DEFAULT_INIT_SHELL="/sbin/openrc-run" for Gentoo and Alpine to ensure the correct shebang is always used regardless of build-time availability of openrc-run.

How Has This Been Tested?

mv /sbin/openrc-run /sbin/openrc-run.old
ebuild /var/db/repos/gentoo/sys-fs/zfs/zfs-2.1.1-r2.ebuild unpack.
Edited /var/tmp/portage/sys-fs/zfs-2.1.1-r2/work/zfs-2.1.1/config/zfs-build.m4 to reflect changes in this PR.
ebuild /var/db/repos/gentoo/sys-fs/zfs/zfs-2.1.1-r2.ebuild merge.
Confirmed correct shebang.
mv /sbin/openrc-run.old /sbin/openrc-run

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.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.