Initialize a socket observer

Property definitions

socket $ SocketObserver :: with_sets
	# Initialize a socket observer
	init with_sets(read: Bool, write: Bool, except: Bool) do
		if read then read_set = new SocketSet
		if write then write_set = new SocketSet
		if except then except_set = new SocketSet
	end
lib/socket/socket.nit:322,2--327,4