bug#28540: eshell/sudo find-file doesn't work as expected with files only readable by root

[Yegor Timoshenko]

To reproduce, M-x eshell and compare behavior of these two commands:

$ ff /sudo::/etc/sudoers
$ sudo ff /etc/sudoers

In the first case the buffer is editable (and can be saved), while in
the latter it is opened read-only. Switching read-only flag with C-x
C-q doesn't help: I can edit the buffer, but not save it (results in
"Doing chmod: operation not permitted" error).

GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2017-09-21 (198ba449845ffa557ac272c3219c703148648f53)

Reproducible in Emacs 25.3 as well.