Implement metacommand 'output display' to show it without checking it; try harder...

Implement metacommand 'output display' to show it without checking it; try harder to stick to the deadlines: childs had the possibility to close their output file descriptor and keep sneaking around without being detected by tesh. There was also a little risk of race condition between the runner thread and the reader one; document the new metacommands

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@3542 48e7efb5-ca39-0410-a469-dd3cf9ba447f