[Imap-uw] MIX flags are reported incorrectly when cleared by another process

[Andy Skalski]

I ran into what looks like a bug in how the MIX (unix) driver handles
system flag changes during a mix_ping().  It doesn't handle flags that
are turned off by another process.

Here's what happens when I open two IMAP connections to the same
mailbox:

A1 FETCH 1 FLAGS
* 1 FETCH (FLAGS ())
A1 OK FETCH completed

        B1 FETCH 1 FLAGS
        * 1 FETCH (FLAGS ())
        B1 OK FETCH completed

A2 STORE 1 +FLAGS \Seen
* 1 FETCH (FLAGS (\Seen))
A2 OK STORE completed

        B2 NOOP
        * 1 FETCH (FLAGS (\Seen))
        B2 OK NOOP completed

A3 STORE 1 -FLAGS \Seen
* 1 FETCH (FLAGS ())
A3 OK STORE completed

        B3 NOOP
        * 1 FETCH (FLAGS (\Seen))
        B3 OK NOOP completed

        B4 FETCH 1 FLAGS
        * 1 FETCH (FLAGS (\Seen))
        B4 OK FETCH completed


Connection "B" correctly reports when the flag is set, but not when it
is cleared.

Andrew Skalski