[VM] VM writes r/o folders

[Julian Bradfield]

VM is behaving badly with read-only folders.

If I visit a folder read-only, I don't expect it to be changed and
written back to disk. However, this happens:

(a) If I visit a folder created by my MTA, VM says "upgrading
    attributes", and then the folder is modified, and VM writes it
    back to disk when I quit. This of course causes corruption if the
    MTA is writing new mail at the same time, since they use different
    locking mechanisms.
(b) If I visit a virtual read-only folder, and then I change the
    attributes of a message (by, say, filing it), the underlying
    folder has its attributes changed and gets written when I quit the
    virtual folder. I don't think this is right either.

My understanding of a read-only folder is that nothing I do to it
should cause any modification of the disk file.