safely: Temporary file name changes.

  * Use CL pathname magic to mess only with the name component of paths
    rather than putting extensions on the end, which seems to confuse
    CMU CL.

  * Use a Scsh-like fresh-name generator where it matters, rather than
    hoping (in the case of move-aside names).  This means generating old
    names later on in the process, but that seems OK.

  * Do the file copy in plain Lisp using read/write-sequence rather than
    Unix hacking.  There's a hack here for CMU CL to use link(2) for
    efficiency's sake, but it's not strictly necessary.

This means that the whole package is, in fact, standard Common Lisp.
Yay.