Implement bookmark copying

This feature allows copying existing bookmarks using

    zfs bookmark fs#target fs#newbookmark

There are some niche use cases for such functionality,
e.g. when using bookmarks as markers for replication progress.

Copying redaction bookmarks produces a normal bookmark that
cannot be used for redacted send (we are not duplicating
the redaction object).

ZCP support for bookmarking (both creation and copying) will be
implemented in a separate patch based on this work.

Overview:

- Terminology:
    - source = existing snapshot or bookmark
    - new/bmark = new bookmark
- Implement bookmark copying in `dsl_bookmark.c`
  - create new bookmark node
  - copy source's `zbn_phys` to new's `zbn_phys`
  - zero-out redaction object id in copy
- Extend existing bookmark ioctl nvlist schema to accept
  bookmarks as sources
  - => `dsl_bookmark_create_nvl_validate` is authoritative
- use `dsl_dataset_is_before` check for both snapshot
  and bookmark sources
- Adjust CLI
  - refactor shortname expansion logic in `zfs_do_bookmark`
- Update man pages
  - warn about redaction bookmark handling
- Add test cases
  - CLI
  - pyyzfs libzfs_core bindings

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #9571

man/man8/zfs-bookmark.8

@@ -29,6 +29,7 @@
 .\" Copyright 2019 Richard Laager. All rights reserved.
 .\" Copyright 2018 Nexenta Systems, Inc.
 .\" Copyright 2019 Joyent, Inc.
+.\" Copyright (c) 2019, 2020 by Christian Schwarz. All Rights Reserved.
 .\"
 .Dd June 30, 2019
 .Dt ZFS-BOOKMARK 8 SMM
@@ -42,14 +43,19 @@
 .It Xo
 .Nm
 .Cm bookmark
-.Ar snapshot bookmark
+.Ar snapshot Ns | Ns Ar bookmark newbookmark
 .Xc
-Creates a bookmark of the given snapshot.
+Creates a new bookmark of the given snapshot or bookmark.
 Bookmarks mark the point in time when the snapshot was created, and can be used
 as the incremental source for a
 .Xr zfs-send 8
 command.
 .Pp
+When creating a bookmark from an existing redaction bookmark, the resulting
+bookmark is
+.Sy not
+a redaction bookmark.
+.Pp
 This feature must be enabled to be used.
 See
 .Xr zpool-features 5

man/man8/zfs.8

@@ -200,7 +200,7 @@ Streams are created using the
 .Xr zfs-send 8
 subcommand, which by default creates a full stream.
 .It Xr zfs-bookmark 8
-Creates a bookmark of the given snapshot.
+Creates a new bookmark of the given snapshot or bookmark.
 Bookmarks mark the point in time when the snapshot was created, and can be used
 as the incremental source for a
 .Nm zfs Cm send