Partially rewrites the Platal engine.

Partially rewrites the Platal engine.

This rewrites aims at providing a better API to:
 * include support for %grp directly in the engine (no more hack required
   in Xnet classes).
 * Factorize code and makes code easier to understand.
 * Improves hook lookup performances (O(strlen(path) instead of
   O(hook_count * strlen(path)).

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>