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.

doc/fixes3-7-0.txt

@@ -1362,6 +1362,7 @@ if confused #loot while on a throne moved whole stack of quivered gold into
 	leading to crash when quiver was subsequently accessed
 when filling quiver slot, don't bother asking "what do you want to ready?" if
 	invent is empty
+no secret doors or corridors on the first two dungeon levels
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

include/extern.h

@@ -1531,6 +1531,7 @@ extern void topologize(struct mkroom *) NONNULLARG1;
 extern void place_branch(branch *, coordxy, coordxy) NO_NNARGS;
 extern boolean occupied(coordxy, coordxy);
 extern int okdoor(coordxy, coordxy);
+extern boolean maybe_sdoor(int);
 extern void dodoor(coordxy, coordxy, struct mkroom *) NONNULLARG3;
 extern void mktrap(int, unsigned, struct mkroom *, coord *) NO_NNARGS;
 extern void mkstairs(coordxy, coordxy, char, struct mkroom *, boolean);

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

src/sp_lev.c

@@ -2498,7 +2498,9 @@ search_door(
 }
 
 /*
- * Dig a corridor between two points.
+ * Dig a corridor between two points, using terrain ftyp.
+ * if nxcor is TRUE, he corridor may be blocked by a boulder,
+ * or just end without reaching the destination.
  */
 boolean
 dig_corridor(
@@ -2545,12 +2547,12 @@ dig_corridor(
 
         crm = &levl[xx][yy];
         if (crm->typ == btyp) {
-            if (ftyp != CORR || rn2(100)) {
+            if (ftyp == CORR && maybe_sdoor(100)) {
+                crm->typ = SCORR;
+            } else {
                 crm->typ = ftyp;
                 if (nxcor && !rn2(50))
                     (void) mksobj_at(BOULDER, xx, yy, TRUE, FALSE);
-            } else {
-                crm->typ = SCORR;
             }
         } else if (crm->typ != ftyp && crm->typ != SCORR) {
             /* strange ... */