Sanitize a bit the <foreach> handling (cosmetics, no bug fixed so far)

Sanitize a bit the <foreach> handling (cosmetics, no bug fixed so far)
Do all the work in the starting tag of <host> and <link> instead of
  saving values in starting tag and using them in closing tag.
This allows to:
 - have only one function in the interception mecanism and no global
   value
 - not have to intercept the closing tags and run directly the
   original functions

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@6769 48e7efb5-ca39-0410-a469-dd3cf9ba447f