org.el (org-set-regexps-and-options): Don't read setup files from read-only buffers

org.el (org-set-regexps-and-options): Don't read setup files from read-only buffers

* org.el (org-set-regexps-and-options): Don't read setup files
from read-only buffers.
(org-file-contents): When no-error is non-nil, throw a less
intrusive message.

This commit prevents Gnus to try reading arbitrary files on your machine
when opening a message with #+SETUPFILE keyword.

In general, it prevents reading a setup file from any read-only buffer.

Thanks to Eric Abrahamsen and Andreas Leha for reporting this.