systemd mount generator and tracking ZEDLET

zfs-mount-generator implements the "systemd generator" protocol,
producing systemd.mount units from the cached outputs of zfs list,
during early boot, integrating with systemd.

Each pool has an indpendent cache of the command

  zfs list -H -oname,mountpoint,canmount -tfilesystem -r $pool

which is kept synchronized by the ZEDLET

  history_event-zfs-list-cacher.sh

Datasets not in the cache will be loaded later in the boot process by
zfs-mount.service, including pools without a cache.

Among other things, this allows for complex mount hierarchies.

Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #7329