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/tran.c

@@ -175,8 +175,7 @@ tran_plane(void)
 	    pr("%s has been hardened and can't be transported\n",
 	       prplane(&plane));
 	    return RET_FAIL;
-	} else if ((plane.pln_flags & PLN_LAUNCHED) &&
-		   (plchr[type].pl_flags & P_O)) {
+	} else if (pln_is_in_orbit(&plane)) {
 	    pr("%s is in space and can't be transported\n",
 	       prplane(&plane));
 	    return RET_FAIL;