zdb: consistent xattr output

When using zdb to output the value of an xattr only interpret it
as printable characters if the entire byte array is printable.
Additionally, if the --parseable option is set always output the
buffer contents as octal for easy parsing.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14830