Distinguish between planes "in orbit" and "launched"

Distinguish between planes "in orbit" and "launched"

Use new pln_is_in_orbit() when we want to test for orbit specifically,
and test PLN_LAUNCHED when we want to test whether the plane not
sitting in the sector (because it is flying).  This distinction is
pointless at this time, because the only way PLN_LAUNCHED gets set is
when a satellite goes into orbit.  It will become useful in a later
commit, which will use PLN_LAUNCHED to mark flying planes.