fix #H7667 - maybe_reset_pick(), other bad context

When deciding whether to discard interrupted lock/unlock context while
changing levels, maybe_reset_pick() checks whether xlock.box is being
carried.  But it was doing so after the old level had been saved and
memory for non-carried container there had been freed.

That led to a couple of other issues.  context.travelcc was using -1
for 'no cached value', but the fields of travelcc have type 'xchar' and
shouldn't be given negative values.  0 should be fine for 'no cache'.

Failed partial restore which occurred after old game's context had been
loaded would begin a new game with old game's stale context.  Restoring
goes out of its way to avoid that for 'flags' but didn't for 'context'.

src/options.c

@@ -1,4 +1,4 @@
-/* NetHack 3.6	options.c	$NHDT-Date: 1543395749 2018/11/28 09:02:29 $  $NHDT-Branch: NetHack-3.6.2-beta01 $:$NHDT-Revision: 1.334 $ */
+/* NetHack 3.6	options.c	$NHDT-Date: 1543972192 2018/12/05 01:09:52 $  $NHDT-Branch: NetHack-3.6.2-beta01 $:$NHDT-Revision: 1.335 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Michael Allison, 2008. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -747,8 +747,6 @@ initoptions_init()
         warnsyms[i] = def_warnsyms[i].sym;
     iflags.bouldersym = 0;
 
-    iflags.travelcc.x = iflags.travelcc.y = -1;
-
     /* for "special achievement" tracking (see obj.h,
        create_object(sp_lev.c), addinv_core1(invent.c) */
     iflags.mines_prize_type = LUCKSTONE;