Fix command abortion after getting player input

The old code didn't honor command abortion at the following prompts:

* arm third argument

* deliver fourth argument (also simplify)

* fire third argument

* fly and recon prompt for carrier to land on: pln_onewaymission()
  treated abort like empty input, which made planes attempt landing in
  the sector.

* lmine second argument

* order d fourth argument

* power c nat(s) argument

* range second argument

* sail second argument

* shutdown both arguments (first one was broken in commit 84cfd670,
  v4.3.10, second one never worked).

* tend third argument

src/lib/subs/plnsub.c

@@ -114,7 +114,10 @@ pln_onewaymission(struct sctstr *target, int *shipno, int *flagp)
     nships = carriersatxy(target->sct_x, target->sct_y, player->cnum);
     if (nships) {
 	for (;;) {
-	    if (!(p = getstarg(0, "Carrier #? ", buf)) || !*p)
+	    p = getstring("Carrier #? ", buf);
+	    if (!p)
+		return -1;
+	    if (!*p)
 		break;
 	    cno = atoi(p);
 	    if (cno < 0