Fix #812: recovered stair dlevel

Stair dlevels weren't being restored with the correct values when
recovered after the game crashed, apparently because they weren't being
reset back to their 'absolute' level from a 'relative' level.  I'm not
totally sure of why this affected only recovered games (maybe that's the
only time when the 'relative' stair values are used?) but this fix seems
to work.

Fixes #812

src/restore.c

@@ -749,7 +749,7 @@ dorecover(NHFILE* nhfp)
     int rtmp;
 
     /* suppress map display if some part of the code tries to update that */
-    g.program_state.restoring = 1;
+    g.program_state.restoring = REST_GSTATE;
 
     get_plname_from_file(nhfp, g.plname);
     getlev(nhfp, 0, (xint8) 0);
@@ -776,6 +776,8 @@ dorecover(NHFILE* nhfp)
     if (rtmp < 2)
         return rtmp; /* dorecover called recursively */
 
+    g.program_state.restoring = REST_LEVELS;
+
     /* these pointers won't be valid while we're processing the
      * other levels, but they'll be reset again by restlevelstate()
      * afterwards, and in the meantime at least u.usteed may mislead
@@ -909,7 +911,8 @@ rest_stairs(NHFILE* nhfp)
         if (nhfp->structlevel) {
             mread(nhfp->fd, (genericptr_t) &stway, sizeof (stairway));
         }
-        if (stway.tolev.dnum == u.uz.dnum) {
+        if (g.program_state.restoring != REST_GSTATE
+            && stway.tolev.dnum == u.uz.dnum) {
             /* stairway dlevel is relative, make it absolute */
             stway.tolev.dlevel += u.uz.dlevel;
         }