Further cleanups:

Further cleanups:
 - more comments
 - Remove a useless argument to xbt_context_stop (it's always
   current_context)
 - group actions of freeing context fields in xbt_context_free (some
   of them where in xbt_context_stop for no good reason)
 - __xbt_context_yield never receive a NULL argument, so change the if
   to an assert
 - plus other minor cleanups

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@4647 48e7efb5-ca39-0410-a469-dd3cf9ba447f