^A/#repeat vs extended commands

Fix '#repeat' for tty; both it and ^A can repeat an extended command.

Fix both for curses; they can repeat an extended command instead of
just repeating the initial '#' to start getting an extended command.

X11 (tested), Qt (tested), and probably Windows GUI (not tested)
behave the same as before:  ^A (or #repeat) after an extended command
just repeats the # to run the dialog to get an extended command.

I hope this introduces fewer bugs than it fixes but I don't think I'd
bet on that....

include/decl.h

@@ -499,6 +499,7 @@ struct cmd {
     const char *alphadirchars; /* same as dirchars if !numpad */
     const struct ext_func_tab *commands[256]; /* indexed by input character */
     char spkeys[NUM_NHKF];
+    char extcmd_char;      /* key that starts an extended command ('#') */
 };