Cantor Seeks To Relocate To Union Sq.
Cantor Fitzgerald, bond broker that lost 658 employees in Sept 11 terrorist attacks, is negotiating to build $100 million headquarters in Union Square, Manhattan, area better known for restaurants and residential buildings than financial services; Howard Lutnick, Cantor's chairman, is seeking tens of millions of dollars in state and federal grants and tax breaks; deal is far from finished but some in neighborhood are already welcoming company; map (M)