Return finer grain errors in libzfs unmount_one

Added errno mappings to unmount_one() in libzfs. Changed do_unmount()
implementation to return errno errors directly like is done for
do_mount() and others.

Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #11681