cp: Read/write 32 KB at a time to go faster :^)

This is a huge speed-up (3x) when copying large files. Ideally this
would be optimized by the kernel somehow, but we're not there yet.

Userland/cp.cpp

@@ -95,7 +95,7 @@ bool copy_file(String src_path, String dst_path, struct stat src_stat, int src_f
     }
 
     for (;;) {
-        char buffer[BUFSIZ];
+        char buffer[32768];
         ssize_t nread = read(src_fd, buffer, sizeof(buffer));
         if (nread < 0) {
             perror("read src");