No secret doors or corridors on the early levels

New players often get stuck on the first level when they can't
find the secret door or corridor.  Make the first two levels
have no such features.

src/mklev.c

@@ -425,6 +425,7 @@ join(register int a, register int b, boolean nxcor)
         gs.smeq[a] = gs.smeq[b];
 }
 
+/* create random corridors between rooms */
 void
 makecorridors(void)
 {
@@ -447,6 +448,7 @@ makecorridors(void)
                 any = TRUE;
             }
     }
+    /* add some extra corridors which may be blocked off */
     if (gn.nroom > 2)
         for (i = rn2(gn.nroom) + 4; i; i--) {
             a = rn2(gn.nroom);
@@ -1599,10 +1601,18 @@ okdoor(coordxy x, coordxy y)
             && !near_door);
 }
 
+/* do we want a secret door/corridor? */
+boolean
+maybe_sdoor(int chance)
+{
+    return (depth(&u.uz) > 2) && !rn2(max(2, chance));
+}
+
+/* create a door at x,y in room aroom */
 void
 dodoor(coordxy x, coordxy y, struct mkroom *aroom)
 {
-    dosdoor(x, y, aroom, rn2(8) ? DOOR : SDOOR);
+    dosdoor(x, y, aroom, maybe_sdoor(8) ? SDOOR : DOOR);
 }
 
 boolean