[Dovecot] Patch for zlib and maildir for 1.0.13

When using the zlib plugin with maildir and copying with hardlinks,
if a compressed message is copied, the 'Z' suffix on the file isn't
copied, so the new message isn't uncompressed when it's fetched.

I wasn't smart enough to figure out a clean way to carry a file
suffix through a copy, so I changed how the zlib-plugin detects if a
message is compressed. This patch peeks at the first two bytes of
the message looking for the zlib header.

-- Richard Platel Tucows