mount: add ReadWriteOnly property to fail on read-only mounts

Systems where a mount point is expected to be read-write needs a way to
fail mount units that fallback as read-only.

Add a property to allow setting the -w option when calling mount(8).

src/shared/bus-unit-util.c

@@ -1436,7 +1436,8 @@ static int bus_append_mount_property(sd_bus_message *m, const char *field, const
 
         if (STR_IN_SET(field, "SloppyOptions",
                               "LazyUnmount",
-                              "ForceUnmount"))
+                              "ForceUnmount",
+                              "ReadwriteOnly"))
                 return bus_append_parse_boolean(m, field, eq);
 
         return 0;