Create an empty snapshot at the start of snapshot list

[slavanap]

Ability to create an empty snapshot as a first one, prepending large snapshot tree.

$ zfs list 
tank@snap1
tank@snap2
tank@snap3

## sample syntax
$ zfs snap --empty tank@empty

$ zfs list 
tank@empty
tank@snap1
tank@snap2
tank@snap3

$ zfs clone -o mountpoint=/mnt tank@empty tank/tmp
$ ls -la /mnt
total 17
drwxrwxr-x  2 root root  2  feb 3 08:47 .
drwxr-x--x 15 root root 24  frb 3 08:47 ..
$

Usage: empty starting point for redactions (zfs redact, zfs send --redact).

How will this feature improve OpenZFS?

Now redaction of first snapshot requires sending full contents in order to be redacted, thus removal of contents from first snap is not supported (makes redacted snapshot unmountable).

Additional context

Would be also useful to have an ability to insert dummy snapshot elsewhere between other snapshots