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/commands/sate.c

@@ -65,7 +65,7 @@ sate(void)
 	return RET_FAIL;
     }
 
-    if (!(plane.pln_flags & PLN_LAUNCHED)) {
+    if (!pln_is_in_orbit(&plane)) {
 	pr("%s isn't in orbit\n", prplane(&plane));
 	return RET_FAIL;
     }