Always use gmail-flag to find 'AllMail' folder.

Because I have a "[Gmail]/All Mail" folder, but my real 'AllMail' folder is
named "[Google Mail]/All Mail", L-P-F looked into the wrong folder. This is
fixed by always using the flaged folder, unless all_mail is set first.

Signed-off-by: Markus Hubig <mhubig@gmail.com>