Add canonical mount options zfs-mount-generator

Add canonical mount options zfs-mount-generator

lib/libzfs/libzfs_mount.c:zfs_add_options provides the canonical
mount options used by a `zfs mount` command. Because we cannot call
`zfs mount` directly from a systemd.mount unit, we mirror that logic
in zfs-mount-generator.

The zed script is updated to cache these properties as well.

Include a mini-tutorial in the manual page, properly substitute
configuration paths in zfs-mount-generator.8.in, and standardize the
Makefile.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #7453