map: added download checkbox

useful for when off the net
mp_menu: added MPMenuChildMessageDialog()
map: added "Show Position" menu item
map: label as WGS84
map: log positions to positions.txt too
map: fill in ocean areas in SRTM with zero
mavfightview: show full mission
don't wait for heartbeat on secondary links on startup
Adding support for the "WAILLY PPM TO USB Adapter"
Tested with a Spektrum DX7s controller and this USB adapter
lib: added handlink of fd close in child processes
link: moved link handling logic to new link module

this allows for add/remove of links on the fly