Kernel: Fix race between accept() and connect().

We had a bug where an accepted socket would appear to be EOF/disconnected
on the accepting side until the connecting side had called attach_fd().

Fix this by adding a new SocketRole::Connecting state.

Kernel/Socket.h

@@ -8,7 +8,7 @@
 #include <Kernel/UnixTypes.h>
 #include <Kernel/KResult.h>
 
-enum class SocketRole { None, Listener, Accepted, Connected };
+enum class SocketRole { None, Listener, Accepted, Connected, Connecting };
 
 class Socket : public Retainable<Socket> {
 public: