Fix regression in %ENUMERATE-DIRECTORIES.

Previously (18a at least), %ENUMERATE-DIRECTORIES would return a path
even if a directory element did not exist.  This behavior is restored.

See email from cmucl-help, Sep 26, 2014.

 * src/code/filesys.lisp:
   * Fix regression.in %ENUMERATE-DIRECTORIES.  Even if the directory
     does not exist, we continue recursing instead of stopping.
 * src/general-info/release-20f.txt:
   * Update
 * tests/filesys.lisp:
   * New file adding tests for UNIX-NAMESTRING.