[Patch #1514543] mailbox (Maildir): avoid losing messages on name clash

Two changes:

Where possible, use link()/remove() to move files into a directory; this
makes it easier to avoid overwriting an existing file.

Use _create_carefully() to create files in tmp/, which uses O_EXCL.
diff --git a/Misc/NEWS b/Misc/NEWS
index d91e64d..d41d7cc 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -140,6 +140,10 @@
 - Bug #1575506: mailbox.py: Single-file mailboxes didn't re-lock
   properly in their flush() method.
 
+- Patch #1514543: mailbox.py: In the Maildir class, report errors if there's 
+  a filename clash instead of possibly losing a message.  (Patch by David
+  Watson.)
+
 - Patch #1514544: mailbox.py: Try to ensure that messages/indexes have
   been physically written to disk after calling .flush() or
   .close(). (Patch by David Watson.)