U.S. readies strikes, Syrians prepare for attack

* U.S. officials say strikes could last days * UN team on second sampling visit after poison gas attack * UN's Ban says inspectors must be given time to work * Russia, China, Iran warn of risks of wider war (Adds Merkel and Cameron call, Boehner comment) By Oliver Holmes and Erika Solomon BEIRUT, Aug 28 (Reuters) - People in Damascus stocked up on supplies on Wednesday and some left homes close