Lua API: Allow WML tags with leading underscore (#3877)

@gfgtdf said that it's more consistent to allow them across the board,
which IMHO is a strong enough argument for the API change in master.

changelog.md

@@ -50,6 +50,7 @@
    * wml.tostring() now outputs a string that can be parsed back to WML without loss of data.
    * Add wml.clone() function that performs a deep copy of a config or vconfig.
    * Organize API functions into several new (sub)modules: gui, wesnoth.units, wesnoth.interface
+   * Allow WML tag names injected with wml.tag to start with underscores.
  ### User Interface
    * Don't show in the sidebar the time of day schedule of a shrouded hex. (issue #3638)
  ### Packaging

data/test/scenarios/test_lua_wml_tagnames.cfg

@@ -56,12 +56,18 @@
                     wml.tostring(table)
                 end))
 
-                -- Tag names can't start with underscores.
+                -- Tag names can start with underscores.
-                assert_equal(false, pcall(function()
+                assert_equal(true, pcall(function()
                     local table = {T._reserved {}}
                     wml.tostring(table)
                 end))
 
+                -- An underscore by itself isn't allowed.
+                assert_equal(false, pcall(function()
+                    local table = {T._ {}}
+                    wml.tostring(table)
+                end))
+
                 -- Commit 13a4822d made the WML parser accept dollar signs in tag names.
                 -- However, they are supposed to be rejected, and this test enforces
                 -- that at least the Lua API rejects them.

src/scripting/lua_common.cpp

@@ -757,7 +757,7 @@ 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) || m[0] == '_') return_misformed();
+		if (!m || !config::valid_tag(m)) return_misformed();
 		lua_rawgeti(L, -2, 2);
 		if (!luaW_toconfig(L, -1, cfg.add_child(m)))
 			return_misformed();