new #overview menu

Require the 'm' prefix to treat #overview as a menu with selectable
entries.  During end-of-game disclosure it shows every level that was
visited, but during play it only shows levels which have annotations
(typically automatically generated ones).  The menu wasn't offering
any chance to add an annotation to levels without such, so force
'm #overview' to show every visited level.  Continue to avoid that
(and also avoid the clutter introduced by menu entry selector letters)
for normal #overview.

src/cmd.c

@@ -2636,9 +2636,12 @@ struct ext_func_tab extcmdlist[] = {
     { '\0',   "optionsfull", "show all option settings, possibly change them",
               doset, IFBURIED | GENERALCMD | CMD_M_PREFIX, NULL },
     /* #overview used to need autocomplete and has retained that even
-       after being assigned to ^O [old wizard mode ^O is now #wizwhere] */
+       after being assigned to ^O [old wizard mode ^O is now #wizwhere];
+       'm' prefix displays overview as a menu where player can choose a
+       level to supply with an annotation */
     { C('o'), "overview", "show a summary of the explored dungeon",
-              dooverview, IFBURIED | AUTOCOMPLETE | GENERALCMD, NULL },
+              dooverview,
+              IFBURIED | AUTOCOMPLETE | GENERALCMD | CMD_M_PREFIX, NULL },
     /* [should #panic actually autocomplete?] */
     { '\0',   "panic", "test panic routine (fatal to game)",
               wiz_panic, IFBURIED | AUTOCOMPLETE | WIZMODECMD, NULL },