Add support to decode a resume token

Adding a new subcommand to zstream called token. This
now allows users to decode a resume token to retrieve the toname
field. This can be useful for tools that need this information.
The syntax works as follows zstream token <resume_token>.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Signed-off-by: Tony Perkins <tperkins@datto.com>
Closes #10558

man/man8/zstream.8

@@ -35,6 +35,9 @@
 .Cm redup
 .Op Fl v
 .Ar file
+.Nm
+.Cm token
+.Ar resume_token
 .Sh DESCRIPTION
 .sp
 .LP
@@ -67,6 +70,12 @@ Implies verbose.
 .El
 .It Xo
 .Nm
+.Cm token
+.Ar resume_token
+.Xc
+Dumps zfs resume token information
+.It Xo
+.Nm
 .Cm redup
 .Op Fl v
 .Ar file