Kernel: Add a writev() syscall for writing multiple buffers in one go.

We then use this immediately in the WindowServer/LibGUI communication in
order to send both message + optional "extra data" with a single syscall.

Kernel/UnixTypes.h

@@ -390,3 +390,8 @@ struct [[gnu::packed]] FarPtr {
     dword offset { 0 };
     word selector { 0 };
 };
+
+struct iovec {
+    void* iov_base;
+    size_t iov_len;
+};