help: re-implement 'help' independent from tree of struct command

The current implementation of "help" related commands is tightly
connected to the tree of struct command.
The TCL commands 'add_usage_text' and 'add_help_text' have to add
fake commands in the tree of struct command to handle the help of
TCL procs.

Move all the help texts in a list accessible from the struct
command_context and register the commands through their full name.
Keep the list sorted alphabetically by the command name, so the
result of commands 'help' and 'usage' will be sorted too.

Remove the associated help and usage during commands un-register,
but call help_del_all_commands() for the text added through TCL
commands 'add_usage_text' and 'add_help_text'.

The resulting help and usage output is not changed by this patch
(tested on all the help and usage strings in current master
branch).

Change-Id: Ifd37bb5bd374cba1a22cd7aac208505b4ae1e6fc
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5670
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>

src/openocd.c

@@ -361,6 +361,7 @@ int openocd_main(int argc, char *argv[])
 	server_free();
 
 	unregister_all_commands(cmd_ctx, NULL);
+	help_del_all_commands(cmd_ctx);
 
 	/* free all DAP and CTI objects */
 	dap_cleanup_all();