New field for replication: "readonly" ("Destination dataset read-only")

Possible values:

"SET" ("Set read-only"): Set all destination datasets to readonly=on after finishing the replication.
"REQUIRE" ("Require read-only"): require all existing destination datasets to have readonly=on property.
"IGNORE" ("Ignore"): avoid this kind of behavior

Default is SET for FreeNAS, REQUIRE for TrueNAS. Also this is only possible values for LEGACY transport.

In form goes before "allow from scratch"

In wizard when selecting "Run Once" we should display checkbox "Set destination dataset to be read-only" and use "SET" if checked (by default) and "IGNORE" if not checked