CL:PROBE-FILE signals an error when passed a directory on CLISP.

Please refer to:

http://clisp.org/impnotes.html#probe-file

Their rationale for this is documented here:

http://clisp.org/impnotes.html#dir-is-not-file

This commit replaces calls to CL:PROBE-FILE with CL-FAD:FILE-EXISTS-P
and CL-FAD:DIRECTORY-EXISTS-P to make clhs work on CLISP.