don't take string-write mutex in read.c:read_token

don't take string-write mutex in read.c:read_token

* libguile/read.c (read_token): Don't take the string-write mutex when
  reading a token into a buffer, because it's assumed that the buffer is
  fresh (not seen by other threads), and a soft port can call a
  procedure that needs the string-write mutex.