Some new features in whitelist and bug fixes related to attachment handling

I made these changes to make it work for some of the requirements I had.
Following are the changes

Fixed: the bug that prefix each attachment file name with message id.

Fixed: the error of that copy attachments of one mail to another if they are processed at the same time.

Fixed: the code to use content id instead of sequence when resolving CID of inline embedded attachment images. 
This fix issues that happen when there are both attachments and embedded images.

New: Support granting sending permission for each mailing list separately.

New: Allow multiple white listed senders to post to list on behalf of a list owner without having list owners credentials.

Use same mechanism that phplist uses to download attachments. Using a file similar to dl.php.
In fact the new file dl_mailtolist.php is a copy of dl.php with some tiny changes.
If needed these changes can directly placed inside dl.php.

Files Modified:
- get_email.class.php
- main.php
New File
- dl_mailtolist.php - this is a modified version of dl.php from phplist.