lower the gravity of the message indicating that setsockopt failed to set the buffer...

lower the gravity of the message indicating that setsockopt failed to set the buffer sizes. It do fail on darwin, but I doubt nowadays that this setting does any good. OSes know better than me what the good buffer size should be. Maybe we should kill that completely

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@6237 48e7efb5-ca39-0410-a469-dd3cf9ba447f