cat: Use a 32 KB I/O buffer here to improve "cat a > b" scenario

This is roughly twice as fast as the old 4 KB buffer size. We still
don't go nearly as fast as "cp", since we don't ftruncate() up front
like "cp" does.

Userland/cat.cpp

@@ -24,7 +24,7 @@ int main(int argc, char** argv)
     }
     for (auto& fd : fds) {
         for (;;) {
-            char buf[4096];
+            char buf[32768];
             ssize_t nread = read(fd, buf, sizeof(buf));
             if (nread == 0)
                 break;