Re: trusted-content seems to have effect only with sources specified.

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> Isn't it obvious that trust should be given to actual files and
>> directories, not links to them?
> Yes, what is not obvious is to think that the problem is caused by
> symbolic links

Currently the error/warning message states the name of the buffer
(which is better because the trust can depend on other things than the
name of the associated file), but it would be helpful to include the
`buffer-file-truename` in the message to guide the user to the right
file/dir name to add to `trusted-content`.

Arguably, in many cases a better UI would be a bit like for
file/dir-local vars where we prompt the user whether to trust or not,
and if yes, whether to do it permanently (i.e. to add an entry to
`trusted-content` and `custom-save` the result).


        Stefan