fix #H30 - rn2(0) from off by 1 bug in special level door creation

From a bug report, the placement
of random doors by the code that loads special levels would attempt to
evaluate rn2(0) and either get a divide by zero crash (normal build) or an
impossible warning (DEBUG enabled when compiing rnd.c, done automatically
when BETA is defined).  The problem was only noticable for random door in
a 1x1 room; none of our distributed levels specify such a thing so regular
users won't have encountered this bug.  It's a one line fix.

     Altar placement in temples also had a quirk of a similar nature.  It
wouldn't trigger rn2(0) problems but would always place the altar to left
of mid-point in rooms with even width and above the center point in ones
with even height.  Now the placement is randomized so that sometimes it'll
be to the right and/or below mid-point in such cases.

     This also simplifies a couple other instances of similar expressions
that I spotted.

src/mklev.c

@@ -1,4 +1,4 @@
-/*	SCCS Id: @(#)mklev.c	3.5	2001/11/29	*/
+/*	SCCS Id: @(#)mklev.c	3.5	2006/01/28	*/
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -83,8 +83,8 @@ xchar xl,yl,xh,yh;
 {
 	register xchar x, y;
 
-	x = (xl == xh) ? xl : (xl + rn2(xh-xl+1));
-	y = (yl == yh) ? yl : (yl + rn2(yh-yl+1));
+	x = rn1(xh - xl + 1, xl);
+	y = rn1(yh - yl + 1, yl);
 	if(okdoor(x, y))
 		goto gotit;