Small patch that prevents displaying a "local echo" of one's own aircraft

Small patch that prevents displaying a "local echo" of one's own aircraft
under some circumstances. The history of this patch is somewhat unclear,
but was brought to my attention by Martin Spott, while preparing for the
Lelystad FSWeekend show. See also my posting on FlightGear devel, on
November 22, 2007 "(Multiplayer Local Echo Patch)", but wasn't committed
then because I/we assumed that had been superseded by other code
modifications. The local echo problem still persists, however, albeit
under specific circumstances. The current patch reportedly prevents this
from happening.