Streamline plist initialization

msl_equip(), find_escorts() and perform_mission() memset() the plist,
then assign to all members but load.  Just zero load instead, like
getilists(), msl_sel() and pln_sel() do.

src/lib/commands/laun.c

@@ -354,7 +354,7 @@ msl_equip(struct plnstr *pp, char mission)
 {
     struct plist pl;
 
-    memset(&pl, 0, sizeof(struct plist));
+    pl.load = 0;
     pl.pcp = plchr + pp->pln_type;
     pl.plane = *pp;
     emp_initque(&pl.queue);