streamline and document helptext mode displays

Most commands are usable only at runtime; so don't bother saying
that, it's noise.  Moreover, tokens like EXEC are cryptic.  Be
more clear: highlight only the commands which may (also) be used
during the config stage, thus matching the docs more closely.
There are

 - Configuration commands (per documentation)
 - And also some commands that valid at *any* time.

Update the docs to note that "help" now shows this mode info.

This also highlighted a few mistakes in command configuration,
mostly commands listed as "valid at any time" which shouldn't
have been.  This just fixes ones I noted when sanity testing.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/server/gdb_server.c

@@ -2272,6 +2272,7 @@ static int gdb_target_start(struct target *target, uint16_t port)
 	return ERROR_OK;
 }
 
+/* FIXME static */
 int gdb_target_add_one(struct target *target)
 {
 	if (gdb_port == 0 && server_use_pipes == 0)
@@ -2420,7 +2421,7 @@ static const struct command_registration gdb_command_handlers[] = {
 	{
 		.name = "gdb_port",
 		.handler = &handle_gdb_port_command,
-		.mode = COMMAND_ANY,
+		.mode = COMMAND_CONFIG,
 		.help = "daemon configuration command gdb_port",
 		.usage = "<port>",
 	},