Allow WML tags with leading underscore (#3877)

@gfgtdf pointed out that the WML parser can deal with such tags after all.
I had missed that when I implemented the check that the game state is
valid WML before saving it (commit 3bc36ef).

Such tags are still disallowed in the Lua API because allowing them would
be an API change.

src/config.cpp

@@ -186,8 +186,8 @@ bool config::valid_tag(config_key_type name)
 	if(name == "") {
 		// Empty strings not allowed
 		return false;
-	} else if(name[0] == '_') {
-		// Underscore can't be the first character
+	} else if(name == "_") {
+		// A lone underscore isn't a valid tag name
 		return false;
 	} else {
 		return std::all_of(name.begin(), name.end(), [](const char& c)

src/scripting/lua_common.cpp

@@ -745,7 +745,7 @@ bool luaW_toconfig(lua_State *L, int index, config &cfg)
 		if (!lua_istable(L, -1)) return_misformed();
 		lua_rawgeti(L, -1, 1);
 		char const *m = lua_tostring(L, -1);
-		if (!m || !config::valid_tag(m)) return_misformed();
+		if (!m || !config::valid_tag(m) || m[0] == '_') return_misformed();
 		lua_rawgeti(L, -2, 2);
 		if (!luaW_toconfig(L, -1, cfg.add_child(m)))
 			return_misformed();