global-autorevert-mode does not preserve read-only status

[Jari Aalto]

Test suite:

    1. C-x C-f /var/log/syslog (or any other growing log file)
    2. That buffer shoudl not be edited, so we want C-x C-q
    3. M-x global-autorevert-mode (start following all logs)

... after a while the log buffer is reverted, but unfortunately the
read only status that the user activated for the buffer is not
preserved after auto reverted has reloaded the buffer.

I think the state should be preserved as user had set it.

Jari