Fix Maildir race

Fix Maildir race

fixes deb#439384

From: martin f krafft
Subject: race condition in Maildir writing

The offlineimap Maildir code checks for file existence and then
opens a file. That's open to a race condition. It's better to open
the file and fail if it already exists. The following patch does
this. It catches OSError 17 (file exists) and re-raises all others.
I'll leave it up to you to decide whether this is appropriate.