(ange-ftp-hash-entry-exists-p)

(ange-ftp-hash-entry-exists-p)
(ange-ftp-file-entry-p, ange-ftp-file-symlink-p): Since the code
has been converted to use hashtables, the relation `nil=none' is
no longer valid, as `nil' is not a hashtable.  This patch tries to
reduce the number of resulting errors.