zfs should optionally send holds

Add -h switch to zfs send command to send dataset holds. If
holds are present in the stream, zfs receive will create them
on the target dataset, unless the zfs receive -h option is used
to skip receive of holds.

Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #7513

include/libzfs.h

@@ -645,6 +645,9 @@ typedef struct sendflags {
 
 	/* only send received properties (ie. -b) */
 	boolean_t backup;
+
+	/* include snapshot holds in send stream */
+	boolean_t holds;
 } sendflags_t;
 
 typedef boolean_t (snapfilter_cb_t)(zfs_handle_t *, void *);
@@ -707,6 +710,12 @@ typedef struct recvflags {
 
 	/* do not mount file systems as they are extracted (private) */
 	boolean_t nomount;
+
+	/* Was holds flag set in the compound header? */
+	boolean_t holds;
+
+	/* skip receive of snapshot holds */
+	boolean_t skipholds;
 } recvflags_t;
 
 extern int zfs_receive(libzfs_handle_t *, const char *, nvlist_t *,