zdb: Add -O option for -r to specify object-id

"zdb -r -O pool/dataset obj-id destination" will copy
the file with object-id obj-id to the named destination;
without -O it'll still be interpreted as a pathname.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Akash B <akash-b@hpe.com>
Signed-off-by: Sean Eric Fagan <sean.fagan@klarasystems.com>
Closes #16307

man/man8/zdb.8

@@ -84,6 +84,11 @@
 .Op Fl K Ar key
 .Ar dataset path destination
 .Nm
+.Fl r
+.Fl O
+.Op Fl K Ar key
+.Ar dataset object-id destination
+.Nm
 .Fl R
 .Op Fl A
 .Op Fl e Oo Fl V Oc Oo Fl p Ar path Oc Ns …
@@ -376,6 +381,12 @@ Specified
 .Ar path
 must be relative to the root of
 .Ar dataset .
+When used with
+.Fl O ,
+the
+.Ar path
+argument is interpreted as an object identifier,
+not a path.
 This option can be combined with
 .Fl v
 for increasing verbosity.