libzfs: add zfs_get_underlying_type. Stop including libzfs_impl.h in cmd

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12116

lib/libzfs/libzfs_dataset.c

@@ -3333,6 +3333,16 @@ zfs_get_type(const zfs_handle_t *zhp)
 	return (zhp->zfs_type);
 }
 
+/*
+ * Returns the type of the given zfs handle,
+ * or, if a snapshot, the type of the snapshotted dataset.
+ */
+zfs_type_t
+zfs_get_underlying_type(const zfs_handle_t *zhp)
+{
+	return (zhp->zfs_head_type);
+}
+
 /*
  * Is one dataset name a child dataset of another?
  *