Do not ignore gmail labels if header appears multiple times

There should be just one header storing gmail labels, but due to a bug,
multiple X-Keywords (or equivalent) headers may be found on the local
messages.

Now we, when extracting the labels from a message, we read all label
headers, instead of just the first one.

This has the consequence that some old labels stored locally in a second
X-Keywords (or third...) header, which effectively was rendered
invisible to offlineimap until now, may pop back up again and be pushed
to gmail. No labels will be removed by the changes in this commit,
though.

Signed-off-by: Abdo Roig-Maranges <abdo.roig@gmail.com>
Delete gmail labels header before adding a new one

This fixes a bug in which a message ended up with multiple gmail labels
header (X-Keywords or so). Fix fix it by removing all labels headers
before adding the updated one.

Signed-off-by: Abdo Roig-Maranges <abdo.roig@gmail.com>
Warn about a tricky piece of code in addmessageheader

As requested by Nicholas Sebrecht.

Signed-off-by: Abdo Roig-Maranges <abdo.roig@gmail.com>