Monster list iterator

Add some basic functions to iterate through the monster list,
ignoring dead monsters. Mainly just to allow splitting up code
into discrete functions.

Not quite happy with the get_iter_mons_xy - should probably have
a pointer to iterator data struct, which gets passed through instead,
but this works for now.

src/display.c

@@ -131,6 +131,7 @@ static void display_warning(struct monst *);
 static int check_pos(int, int, int);
 static int get_bk_glyph(xchar x, xchar y);
 static int tether_glyph(int, int);
+static void mimic_light_blocking(struct monst *);
 #ifdef UNBUFFERED_GLYPHINFO
 static glyph_info *glyphinfo_at(xchar, xchar, int);
 #endif
@@ -1304,6 +1305,17 @@ see_monsters(void)
         newsym(u.ux, u.uy);
 }
 
+static void
+mimic_light_blocking(struct monst *mtmp)
+{
+    if (mtmp->minvis && is_lightblocker_mappear(mtmp)) {
+        if (See_invisible)
+            block_point(mtmp->mx, mtmp->my);
+        else
+            unblock_point(mtmp->mx, mtmp->my);
+    }
+}
+
 /*
  * Block/unblock light depending on what a mimic is mimicing and if it's
  * invisible or not.  Should be called only when the state of See_invisible
@@ -1312,18 +1324,7 @@ see_monsters(void)
 void
 set_mimic_blocking(void)
 {
-    register struct monst *mon;
-
-    for (mon = fmon; mon; mon = mon->nmon) {
-        if (DEADMONSTER(mon))
-            continue;
-        if (mon->minvis && is_lightblocker_mappear(mon)) {
-            if (See_invisible)
-                block_point(mon->mx, mon->my);
-            else
-                unblock_point(mon->mx, mon->my);
-        }
-    }
+    iter_mons(mimic_light_blocking);
 }
 
 /*