platform: consider RTNH_F_ONLINK onlink flag for IPv4 routes

The "onlink" flag for IPv4 routes is part of the route ID.
Consider it in nm_platform_ip4_route_cmp().

Also, allow configuring the flag when adding a route.

Note that for IPv6, the onlink flag is still ignored.
Pretty much like kernel does.

src/platform/nm-linux-platform.c

@@ -2690,7 +2690,9 @@ _nl_msg_new_route (int nlmsg_type,
 		             ? nm_platform_route_scope_inv (obj->ip4_route.scope_inv)
 		             : RT_SCOPE_NOWHERE,
 		.rtm_type = RTN_UNICAST,
-		.rtm_flags = 0,
+		.rtm_flags = obj->ip_route.r_rtm_flags & (is_v4
+		                                          ? (unsigned) (RTNH_F_ONLINK)
+		                                          : (unsigned) 0),
 		.rtm_dst_len = obj->ip_route.plen,
 		.rtm_src_len = is_v4
 		               ? 0