proxy: fix the marshalling of the UDP frames
proxy: fix the marshalling of the UDP frames

- the initial length field should be the total length of the whole
  frame including the variable length field and including the length
  field
- when unmarshalling, return the number of bytes of payload actually
  unmarshalled and not the size of the unmarshal buffer

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit