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

include/sys/fs/zfs.h

@@ -1310,6 +1310,8 @@ typedef enum zfs_ioc {
  * not described precisely by generic errno codes.
  *
  * These numbers should not change over time. New entries should be appended.
+ *
+ * (Keep in sync with contrib/pyzfs/libzfs_core/_constants.py)
  */
 typedef enum {
 	ZFS_ERR_CHECKPOINT_EXISTS = 1024,
@@ -1327,6 +1329,7 @@ typedef enum {
 	ZFS_ERR_SPILL_BLOCK_FLAG_MISSING,
 	ZFS_ERR_UNKNOWN_SEND_STREAM_FEATURE,
 	ZFS_ERR_EXPORT_IN_PROGRESS,
+	ZFS_ERR_BOOKMARK_SOURCE_NOT_ANCESTOR,
 } zfs_errno_t;
 
 /*