bug#15866: Gnutls elisp code doesn't properly check for file existence

[Stefan Monnier]

>> As I wrote elsewhere, I agree that gnutls.el should ignore file
>> handlers when it looks for certificate files.

> The canonical way to ignore any file name handler is to prefix the file
> name with "/:", see (info "(emacs)Quoted File Names")

That's the canonical way *for the user* to write a file that bypasses
file name handlers (useful when you want to access a local file named
/ssh:foo:bar, for example).

Whereas in gnutls.el the issue is for the *code* to check that the
libgnutls C code (which will not have access to the file name handlers)
will be able to use the file name provided by the user.


        Stefan