Compute closest approach to landed targets properly.

Previously, the target approach variables appeared as NaN, and
the orbital display freaked out and stopped showing anything.

To avoid this, first compute the correct approach time by emulating
the planet rotation, then create a circular pseudo orbit that passes
through the target at the correct time for other code to use.
Fix rendering of post-node orbits on the ScanSat screen.

The code should still use the current time as the base for planet
rotation compensation, instead of the node time. Otherwise nodes
and the subsequent orbit are shifted out of place proportional to
time until node.
Merge pull request #104 from angavrilov/master

Improve usability for landing at a specific location.

...Am I correct in thinking this closes #81 ?