* President unharmed in attack
* Somali Islamists claim responsibility
* Attack is one of several in recent weeks (Adds more details of attack)
By Abdi Sheikh
MOGADISHU, Feb 21 (Reuters) - Al Qaeda-linked militants al Shabaab attacked the Somali presidential palace compound on Friday, blasting through a gate with a car bomb and engaging in a fierce gun battle with African peacekeepers, ...