Kernel: Use Userspace<T> for the sendto syscall, and Socket implementation

Note that the data member is of type ImmutableBufferArgument, which has
no Userspace<T> usage. I left it alone for now, to be fixed in a future
change holistically for all usages.

Kernel/API/Syscall.h

@@ -290,7 +290,7 @@ struct SC_sendto_params {
     int sockfd;
     ImmutableBufferArgument<void, size_t> data;
     int flags;
-    const sockaddr* addr;
+    Userspace<const sockaddr*> addr;
     socklen_t addr_length;
 };