bug#17425: 24.4.50; tramp-cleanup-this-connection on push-button

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> - There's a buffer with an existing remote default-directory, connection
>   established.
> - No problem to run browse-url in that buffer (the example was an
>   rcirc buffer). It doesn't hurt, that default-directory is remote.
> - After a while, the remote host wasn't available any more, for whatever
>   reason.
> - Now browse-url fails, because it checks file-directory-p and
>   file-readable-p. Both operations try to access the remote
>   default-directory.

If the process is supposed to run locally, then the right test should be
based on unhandled-file-name-directory (that's what it's for).  If the
process is supposed to be run "on the host referred to be by
default-directory", then re-connecting is the right thing to do.

> If the check would be replaced file-accessible-directory-p, and this
> operation doesn't try to reopen a stalled connection, it would be
> sufficient for that use case.

file-accessible-directory-p is really just another test like
file-directory-p: it should setup a connection if needed.


        Stefan