LocalSocket: Fix mismatch between can_write() and write() logic.

can_write() was saying yes in situations where write() would overflow the
internal buffer. This patch adds a has_attached_peer() helper to make it
easier to understand what's going on in these functions.

Kernel/Net/LocalSocket.h

@@ -25,6 +25,7 @@ public:
 private:
     explicit LocalSocket(int type);
     virtual bool is_local() const override { return true; }
+    bool has_attached_peer(const FileDescriptor&) const;
 
     RetainPtr<FileDescriptor> m_file;