- Don't add the media-specific header until right before transmission

 - Don't add the media-specific header until right before transmission
 - Previously we would store the largest media-specific header size and add that value when creating a packet
 - Makes loopback packets smaller because they have no need for a media-specific header
 - Would fix packet corruption if interfaces with different media-specific headers were installed (if we supported that)
 - Makes adding support for other media types easier

svn path=/trunk/; revision=43287