Add display of checksums to zdb -R

The function zdb_read_block (zdb -R) was always intended to have a :c 
flag which would read the DVA and length supplied by the user, and 
display the checksum. Since we don't know which checksum goes with 
the data, we should calculate and display them all.

For each checksum in the table, read in the data at the supplied 
DVA:length, calculate the checksum, and display it. Update the man 
page and create a zfs test for the new feature.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #9607

man/man8/zdb.8

@@ -249,6 +249,8 @@ and, optionally,
 .Bl -tag -compact -width "b offset"
 .It Sy b Ar offset
 Print block pointer
+.It Sy c
+Calculate and display checksums
 .It Sy d
 Decompress the block. Set environment variable
 .Nm ZDB_NO_ZLE