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

lib/libzfs/os/freebsd/libzfs_zmount.c

@@ -128,8 +128,9 @@ do_mount(zfs_handle_t *zhp, const char *mntpt, char *opts, int flags)
 int
 do_unmount(const char *mntpt, int flags)
 {
-
-	return (unmount(mntpt, flags));
+	if (unmount(mntpt, flags) < 0)
+		return (errno);
+	return (0);
 }
 
 int