Fix UNIXSocket#send_io and recv_io types

[pocke]

This PR fixes the method type definitions for UNIXSocket#{send_io,recv_io}.

send_io

https://docs.ruby-lang.org/en/3.3/UNIXSocket.html#method-i-send_io

send_io actually receives an IO, but the type definition only accepts BasicSocket, which is a subclass of IO. So I fixed it to allow IO.

recv_io

https://docs.ruby-lang.org/en/3.3/UNIXSocket.html#method-i-recv_io

It's a bit complex. It has three patterns.

• Without argument, it returns an IO.
• With the Integer class or nil, it returns an Integer.
• With any object having for_fd method, it calls this method and returns the returned value of for_fd.
  • The first argument of for_fd is the file descripter. The second argument is mode, and the second argument of recv_io.

[pocke]

The argument is rough.

Actually, the definition should be the following:

interface _ForFd[MODE, RET]
  def for_fd: (Integer fd, ?MODE) -> RET
end

But the definition is wrong because the following class does not match the interface.

class ForFd
  def self.for_fd: (Integer fd) -> instance
end

I think it's difficult to define strict type, so ? is acceptable for this case.