Partial fix of GRAS: gras-ping-sg now works again

Partial fix of GRAS: gras-ping-sg now works again
* Make the following function specific to the plugins:
  - gras_socket_my_port(sock)
  - gras_socket_peer_port(sock)
  - gras_socket_peer_name(sock)
  - gras_socket_peer_proc(sock)
  - gras_socket_peer_proc_set(sock,peer_proc)
* In SG plugin, implement them properly knowing that the same socket
  object gets shared between peers
* In TCP plugin, use the previous behavior
* In file plugin, use THROW_UNIMPLEMENTED

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@8545 48e7efb5-ca39-0410-a469-dd3cf9ba447f