initramfs: fixes for (debian) initramfs

* contrib/initramfs: include /etc/default/zfs and /etc/zfs/zfs-functions
At least debian needs /etc/default/zfs and /etc/zfs/zfs-functions for
its initramfs. Include both in build when initramfs is configured.

* contrib/initramfs: include 60-zvol.rules and zvol_id
Include 60-zvol.rules and zvol_id and set udev as predependency instead
of debians zdev. This makes debians additional zdev hook unneeded.

* Correct initconfdir substitution for some distros
Not every Linux distro is using @sysconfdir@/default but @initconfdir@
which is already determined by configure. Let's use it.

* systemd: prevent possible conflict between systemd and sysvinit
Systemd will not load a sysvinit service if a unit exists with the same
name. This prevents conflicts between sysvinit and systemd.
In ZFS there is one sysvinit service that does not have a systemd
service but a target counterpart, zfs-import.target.
Usually it does not make any sense to install both but it is possisble.
Let's prevent any conflict by masking zfs-import.service by default.
This does not harm even if init.d/zfs-import does not exist.

Reviewed-by: Chris Wedgwood <cw@f00f.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Tested-by: Alex Ingram <reimu@reimuhakurei.net>
Tested-by: Dreamcat4 <dreamcat4@gmail.com>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Closes #7904
Closes #9089

etc/systemd/system/Makefile.am

@@ -31,5 +31,9 @@ $(systemdunit_DATA) $(systemdpreset_DATA):%:%.in
 		-e 's,@sysconfdir\@,$(sysconfdir),g' \
 		$< >'$@'
 
+install-data-hook:
+	$(MKDIR_P) "$(DESTDIR)$(systemdunitdir)"
+	ln -s /dev/null "$(DESTDIR)$(systemdunitdir)/zfs-import.service"
+
 distclean-local::
 	-$(RM) $(systemdunit_DATA) $(systemdpreset_DATA)