Ensure that zfs diff prints unicode safely.

In the stream_bytes() library function used by `zfs diff`, explicitly
cast each byte in the input string to an unsigned character so that the
Linux fprintf() correctly escapes to octal and does not mangle the output.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1172
Fix quoting error in unmount command

A misplaced single quote caused the umount command to fail with a
syntax error when unmounting snapshots under the .zfs/snapshot
control directory.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1210