zmodem file transfer doesn't work over mosh

sz/rz commands don't work through mosh and it would be nice if they could.

consider the following use-case:

$ screen      # on local laptop,

press ^a then enter :zmodem catch to enable zmodem receive on the laptop.

(screen)$ mosh host1

host1$ ssh host2

host2$ sz file.c

i think support could be easier than other byte-oriented protocols, because:

1. zmodem frames are very easy to recognise (in the terminal parser and keyboard input)
2. zmodem frames are small (1k in traditional; 8k in lszrz a popular zmodem implementation), so they fit inside (fragmented) a udp packet and if packets are lost, zmodem can shrink the window
3. zmodem retries automatically, so packet delivery isn't required