lua: make add_key_binding() rotate optional arguments correctly

add_key_binding() makes the name argument optional (in weird Lua
fashion), which did not work if there were additional arguments. So
there is no way to avoid specifying a name while passing a rp argument.
Fix this, declare this way of skipping the argument as deprecated, and
allow passing name=nil as the preferred way to skip the name argument.

DOCS/man/lua.rst

@@ -240,7 +240,9 @@ The ``mp`` module is preloaded, although it can be loaded manually with
     an example). The name should be unique across other bindings in the same
     script - if not, the previous binding with the same name will be
     overwritten. You can omit the name, in which case a random name is generated
-    internally.
+    internally. (Omitting works as follows: either pass ``nil`` for ``name``,
+    or pass the ``fn`` argument in place of the name. The latter is not
+    recommended and is handled for compatibility only.)
 
     The last argument is used for optional flags. This is a table, which can
     have the following entries:

player/lua/defaults.lua

@@ -157,8 +157,12 @@ end
 
 local function add_binding(attrs, key, name, fn, rp)
     rp = rp or ""
-    if (type(name) ~= "string") and (not fn) then
+    if (type(name) ~= "string") and (name ~= nil) then
+        rp = fn
         fn = name
+        name = nil
+    end
+    if name == nil then
         name = reserve_binding()
     end
     local repeatable = rp == "repeatable" or rp["repeatable"]