move generation of lon/lat string representations here from cockpit.cxx.

move generation of lon/lat string representations here from cockpit.cxx.
Thanks to tied functions, this is only executed when the node is read. This
will be done by the old & new HUD code, the latter of which won't have a
special lon/lat mode at all. Instead it will be regular labels that point
to these properties for displaying lon/lat.
   I would have liked to avoid the duplication of code (lon and lat being
basically the same thing), and to avoid using static buffers and all, but
... if anyone wants to make it prettier, go ahead.
   The format is controlled by /sim/lon-lat-format (will be changed if I
find a better place).