- Do not close sockets closed on other side: user may keep references to them.

  Instead, mark them as invalid and ignore them.
- Do raise a timeout when a poll fails (timeout=0, but no message here)
- New function: gras_trp_socketset_dump to display the content of the socket
  set.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@2508 48e7efb5-ca39-0410-a469-dd3cf9ba447f