BENTONVILLE, Arkansas/BOSTON (Reuters) – Even as Wal-Mart Stores Inc showed unusual openness to outsiders’ complaints by taking steps including raising wages, it has been trying to slam the door on a shareholder proposal that could weaken the founding Walton family’s power over its board. The investors are pushing hard for changes in how the world’s No. 1 retailer governs itself, seeking to install an independent chairman. Crucial to the debate will be the stance of BlackRock Inc, Wal-Mart’s No.