Add zdb -r <dataset> <object-id | file> <output>

While you can use zdb -R poolname vdev:offset:[<lsize>/]<psize>[:flags] 
to extract individual DVAs from a vdev, it would be handy for be able 
copy an entire file out of the pool.

Given a file or object number, add support to copy the contents to a 
file. Useful for debugging and recovery.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes #11027

man/man8/zdb.8

@@ -15,7 +15,7 @@
 .\" Copyright (c) 2017 Lawrence Livermore National Security, LLC.
 .\" Copyright (c) 2017 Intel Corporation.
 .\"
-.Dd April 14, 2019
+.Dd October 7, 2020
 .Dt ZDB 8 SMM
 .Os
 .Sh NAME
@@ -60,6 +60,9 @@
 .Fl O
 .Ar dataset path
 .Nm
+.Fl r
+.Ar dataset path destination
+.Nm
 .Fl R
 .Op Fl A
 .Op Fl e Oo Fl V Oc Op Fl p Ar path ...
@@ -274,6 +277,19 @@ must be relative to the root of
 This option can be combined with
 .Fl v
 for increasing verbosity.
+.It Fl r Ar dataset path destination
+Copy the specified
+.Ar path
+inside of the
+.Ar dataset
+to the specified destination.
+Specified
+.Ar path
+must be relative to the root of
+.Ar dataset .
+This option can be combined with
+.Fl v
+for increasing verbosity.
 .It Xo
 .Fl R Ar poolname vdev Ns \&: Ns Ar offset Ns \&: Ns Ar [<lsize>/]<psize> Ns Op : Ns Ar flags
 .Xc