lua: implement input_enable_section/input_disable_section via commands

Removes some more internal API calls from the Lua scripting backend.
Which is good, because ideally the scripting backend would use libmpv
functions only.

One awkwardness is that mouse sections are still not supported by the
public commands (and probably will never), so flags like allow-hide-
cursor make no sense to an outside user.

Also, the way flags are passed to the Lua function changes. But that's
ok, because they're only undocumented internal functions, and not
supposed to be used by script users. osc.lua only does due to historical
reasons.

player/lua/osc.lua

@@ -1935,7 +1935,7 @@ end
 
 function do_enable_keybindings()
     if state.enabled then
-        mp.enable_key_bindings("showhide", "allow-vo-dragging|allow-hide-cursor")
+        mp.enable_key_bindings("showhide", "allow-vo-dragging+allow-hide-cursor")
     end
 end