safely.lisp: More on the CLisp RENAME-FILE mess.

The :IF-EXISTS option is only in very new versions of CLisp.  It turns
out that the bizarre double-pathname-merging bug in POSIX:COPY-FILE
doesn't cause trouble if the source is absolute (unsurprising, in
retrospect) so I could use TRUENAME to dig my way out of this mess; but
COPY-FILE will clobber the target file before looking to see whether the
source succeeds (unless persuaded otherwise with :IF-EXISTS, which only
works if RENAME-FILE understands that option).

Give up, and use rename(2) through the FFI.