Want 'zfs send -b'

This change implements 'zfs send -b' which can be used to send only
received property values whether or not they are overridden by local
settings.

This can be very useful during "restore" operations from a backup pool
because it allows to send only the property values originally sent
from the backup source, even though they were later modified on the
destination either by a 'zfs set' operation, explicit 'zfs inherit' or
overridden during the receive process via 'zfs receive -o|-x'.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7156

include/libzfs.h

@@ -673,6 +673,9 @@ typedef struct sendflags {
 
 	/* raw encrypted records are permitted */
 	boolean_t raw;
+
+	/* only send received properties (ie. -b) */
+	boolean_t backup;
 } sendflags_t;
 
 typedef boolean_t (snapfilter_cb_t)(zfs_handle_t *, void *);