* Infighting between hardline Islamist rebels intensifies
* Killing likely to provoke revenge attacks
* Death of al Qaeda fighter a gift to US, rebel says (Adds detail on al-Soury)
By Mariam Karouny
BEIRUT, Feb 23 (Reuters) - A Syrian rebel commander who fought alongside al Qaeda founder Osama bin Laden and was close to its current chief, Ayman al-Zawahiri, was killed by a suicide ...