systemd: read initconfdir

Systemd units do not read @initconfdir@ but refer to variables defined
there, also a minor fixup in zfs-scrub service file.

Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Krzysztof Piecuch <piecuch@kpiecuch.pl>
Closes #12946