Allow the current message to not be in the current filter

[andersk@mit.edu]

Currently when you switch to a filter that doesn’t include the current message, we move the nearest message in the new filter. This means that switching back to the old filter may leave you on a different message.

We should allow the message to remain selected even if it is not in the new filter. This could be indicated in the UI with an arrow to a blank line; or the arrow could just pretend to be on the nearest message in the filter even though it isn’t.

(As a happy side effect, this would do away with the ugly cachedmsgid logic that’s currently causing crashes. It also gets you followlast for free if you set curmsg = INT_MAX.)