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/sp_lev.c

@@ -1,4 +1,4 @@
-/*	SCCS Id: @(#)sp_lev.c	3.5	2006/01/02	*/
+/*	SCCS Id: @(#)sp_lev.c	3.5	2006/01/28	*/
 /*	Copyright (c) 1989 by Jean-Christophe Collet */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -620,7 +620,8 @@ struct mkroom *broom;
 		dpos = dd->pos;
 		if (dpos == -1)	/* The position is RANDOM */
 		    dpos = rn2((dwall == W_WEST || dwall == W_EAST) ?
-			    (broom->hy - broom->ly) : (broom->hx - broom->lx));
+			       (broom->hy - broom->ly + 1) :
+			       (broom->hx - broom->lx + 1));
 
 		/* Convert wall and pos into an absolute coordinate! */