refactor command registration

Refactors the command registration to use helpers to simplify the code.
The unregistration routines were made more flexible by allowing them
to operate on a single command, such that one can remove all of a
commands children in one step (perhaps before adding back a 'config'
subcommand that allows getting the others back).  Eliminates a bit
of duplicated code and adds full API documentation for these routines.

src/openocd.c

@@ -278,7 +278,7 @@ int openocd_main(int argc, char *argv[])
 	httpd_stop();
 #endif
 
-	unregister_all_commands(cmd_ctx);
+	unregister_all_commands(cmd_ctx, NULL);
 
 	/* free commandline interface */
 	command_done(cmd_ctx);