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.

src/lib/subs/plnsub.c

@@ -1236,6 +1236,13 @@ pln_mobcost(int dist, struct plnstr *pp, int flags)
     return ldround(cost * dist / pln_range_max(pp) + 5, 1);
 }
 
+int
+pln_is_in_orbit(struct plnstr *pp)
+{
+    return (plchr[pp->pln_type].pl_flags & (P_M | P_O)) == P_O
+	&& (pp->pln_flags & PLN_LAUNCHED);
+}
+
 /*
  * Set PP's tech to TLEV along with everything else that depends on it.
  */