Remove the per dungeon level door limit

Number of doors in a room-and-corridor style level was fixed
at 120; now the doors-array is dynamically allocated when needed.

Breaks saves and bones.

include/decl.h

@@ -817,7 +817,8 @@ struct instance_globals {
     char toplines[TBUFSZ];
     coord bhitpos; /* place where throw or zap hits or stops */
     boolean in_steed_dismounting;
-    coord doors[DOORMAX];
+    int doors_alloc; /* doors-array allocated size */
+    coord *doors; /* array of door locations */
     struct menucoloring *menu_colorings;
     schar lastseentyp[COLNO][ROWNO]; /* last seen/touched dungeon typ */
     struct spell spl_book[MAXSPELL + 1];