ffap fails with non-ASCII file names

[Dave Love]

`ffap' fails with non-ASCII names due to the value of
`ffap-string-at-point-mode-alist' (at least).  It's not trivial to fix
by replacing the `a-zA-Z' pattern by `[:alpha:]' since it's used by
skip-chars-{fore,back}ward, where you can't use char classes.

Also, it looks as though it isn't compatible with thingatpt's
treatment of file names.  I guess they should be made consistent if
there's a good reason ffap can't use thingatpt (for files and other
things, like URLs).

It's probably worth someone going through the sources again to fix
instances of `a-z' where appropriate.  (I don't remember why I didn't
sort out ffap when I did that before.)