rmail-message-filter

[Glenn Morris]

rmail-message-filter is an option in rmail.el. Post-mbox, it does nothing.
Previously, it was called:

 i) the very first time a message was shown.
ii) when toggling the headers to the normal state, every time.

It's not clear what it should do now, since there is nothing that
records when a message is shown for the first time (unseen is not
necessarily the same thing).

Should it do something (every time a message is shown? when the unseen
attribute is removed? when new mail is gotten), or be removed?