Move dracut specifics to dracut module

Dracut depends on the environment variable BOOTFS to be set after pool
import. This dracut specific systemd ExecStartPost command should not be
called for any non-dracut systems, so let's move it to a static systemd
unit that.

Reviewed-by: Manuel Amador (Rudd-O) <rudd-o@rudd-o.com>
Reviewed-by: Matthew Thode <prometheanfire@gentoo.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Closes #8510

etc/systemd/system/zfs-import-cache.service.in

@@ -13,7 +13,6 @@ ConditionPathExists=@sysconfdir@/zfs/zpool.cache
 Type=oneshot
 RemainAfterExit=yes
 ExecStart=@sbindir@/zpool import -c @sysconfdir@/zfs/zpool.cache -aN
-ExecStartPost=/bin/sh -c "/bin/systemctl set-environment BOOTFS=$(@sbindir@/zpool list -H -o bootfs | grep -m1 -v '^-$')"
 
 [Install]
 WantedBy=zfs-import.target

etc/systemd/system/zfs-import-scan.service.in

@@ -12,7 +12,6 @@ ConditionPathExists=!@sysconfdir@/zfs/zpool.cache
 Type=oneshot
 RemainAfterExit=yes
 ExecStart=@sbindir@/zpool import -aN -o cachefile=none
-ExecStartPost=/bin/sh -c "/bin/systemctl set-environment BOOTFS=$(@sbindir@/zpool list -H -o bootfs | grep -m1 -v '^-$')"
 
 [Install]
 WantedBy=zfs-import.target