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

man/man8/zfs.8

@@ -194,7 +194,7 @@
 .Ar snapshot bookmark
 .Nm
 .Cm send
-.Op Fl DLPRcenpvw
+.Op Fl DLPRbcenpvw
 .Op Oo Fl I Ns | Ns Fl i Oc Ar snapshot
 .Ar snapshot
 .Nm
@@ -3321,7 +3321,7 @@ feature.
 .It Xo
 .Nm
 .Cm send
-.Op Fl DLPRcenpvw
+.Op Fl DLPRbcenpvw
 .Op Oo Fl I Ns | Ns Fl i Oc Ar snapshot
 .Ar snapshot
 .Xc
@@ -3417,6 +3417,14 @@ See
 for details on ZFS feature flags and the
 .Sy embedded_data
 feature.
+.It Fl b, -backup
+Sends only received property values whether or not they are overridden by local
+settings, but only if the dataset has ever been received. Use this option when
+you want
+.Nm zfs Cm receive
+to restore received properties backed up on the sent dataset and to avoid
+sending local settings that may have nothing to do with the source dataset,
+but only with how the data is backed up.
 .It Fl c, -compressed
 Generate a more compact stream by using compressed WRITE records for blocks
 which are compressed on disk and in memory