Allow zfs to send replication streams with missing snapshots

A tentative implementation and discussion was done in #5285.
According to it a send --skip-missing|-s flag has been added.
In a replication stream, when there are snapshots missing in
the hierarchy, if -s is provided print a warning and ignore
dataset (and its children) instead of throwing an error

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pablo Correa Gómez <ablocorrea@hotmail.com>
Closes #11710

include/libzfs.h

@@ -666,6 +666,9 @@ typedef struct sendflags {
 	/* recursive send  (ie, -R) */
 	boolean_t replicate;
 
+	/* for recursive send, skip sending missing snapshots */
+	boolean_t skipmissing;
+
 	/* for incrementals, do all intermediate snapshots */
 	boolean_t doall;