Reintroduce raw sockets.

Reintroduce raw sockets.
 Created by gras_socket_{client,server}_ext;used with gras_raw_{send,recv}
 It should allow to kill the last bits of gras first version.

 This is not completely satisfactory yet (dupplicate code with
 chunk_{send,recv}; a bit out of the plugin mecanism), but it should work.

Simplify transport plugin interface by not passing any argument to _server
 and _client, but embeeding them in the socket struct directly.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@437 48e7efb5-ca39-0410-a469-dd3cf9ba447f