Adding some UDP outputs causes some packets to not be forwarded

I recently updated my MAVProxy, pymavlink, dronekit, and ArduPlane.

Adding some outputs (via output add or by command line argument --out=) causes some packets to not be sent.

Specifically, I have an Ubuntu VM set up with IP addresses 127.0.0.1 and 10.0.2.15. I launch MAVProxy to host a Plane SITL using

` sim_vehicle.py -I2 --console --map --aircraft test`

I can connect to it from Dronekit on 127.0.0.1:14570 and can observe that sending rc commands into MAVproxy (like 'rc 1 2000') works and I receive the updated information in Dronekit.

I can `output add udp:10.0.2.255:14550` with no ill affects.

If I then add an output to my host machine (Windows 7 x64) using
`output add udp:10.29.94.22:14550`

I can connect to this from MissionPlanner to this port and receive position, attitude, mode, etc, but not the 'rcin' values. If I change the rc values (using 'rc 1 1000') in MAVlink, the updated values are no longer reflected in DroneKit.

I do know that this setup worked using an old versions of various software (DroneKit, ArduPlane, pymavlink, MAVProxy (latest as of approximately July 24, 2017
I have attached a TLOG file from the host machine to hopefully help diagnose these packets.

[2018-08-08 14-24-48.zip](https://github.com/ArduPilot/MAVProxy/files/2271661/2018-08-08.14-24-48.zip)

Reproduce the report with `sim_vehicle.py -I2 --console --map --aircraft test`, first using `output add udp:10.0.2.255:14550` and then adding `udp:10.29.94.22:14550`. Compare the packets received by MissionPlanner and DroneKit, using the attached TLOG as evidence, and trace why `rcin` values stop being forwarded. Done means both outputs continue receiving the expected packet types and RC updates remain visible in DroneKit.

Written by the indexing model from the issue text.