systemd zfs-import.target and documentation

zfs-import-{cache,scan}.service must complete before any mounting of
filesystems can occur. To simplify this dependency, create a target
that is reached After (in the systemd sense) the pool is imported.

Additionally, recommend that legacy zfs mounts use the option

x-systemd.requires=zfs-import.target

to codify this requirement.

Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #6764