Make passing paradrop & mine cargo to pln_arm() & friends optional

These missions imply the cargo type, just like bombing missions.  Use
the implied type instead of cargo type parameter ip there.  Parameter
ip is now optional except for missions 't' (transport) and 'd' (drop).

Simplify para() not to pass the optional cargo type.  Leave drop()
alone, because always passing the type is simpler there.

src/lib/commands/para.c

@@ -90,7 +90,7 @@ para(void)
     /*
      * now arm and equip the bombers, transports, whatever.
      */
-    pln_arm(&bomb_list, 2 * ap_to_target, 'a', &ichr[I_MILIT]);
+    pln_arm(&bomb_list, 2 * ap_to_target, 'a', NULL);
     if (QEMPTY(&bomb_list)) {
 	pr("No planes could be equipped for the mission.\n");
 	return RET_FAIL;