Add `ncl_not_using_nextclient` forward

[Xelson]

Add new ncl_not_using_nextclient forward which ensures that player's client will not be able to pass verification and does not declare that it's nextclient as the opposite of ncl_client_api_ready forward

Use cases:

• prevent non-nextclient users to join the server
• logging player's connections

[nrealz0r]

there is already this

• if(ncl_is_next_client(id)) - when player is with nextlient
• if(!ncl_is_next_client(id)) - when player is not with nextlient

there are diference between !
so you can use this for your own in plugin. no need add in API

[Psycrow101]

@nrealz0r ncl_not_using_nextclient is a forward function, not a native function

[nrealz0r]

@Psycrow101 oh i didn't see..