/**
 * Build a Bridge
 * @param end_tile end tile
 * @param flags type of operation
 * @param p1 various bitstuffed elements
 * - p1 = (bit  0-23) - start tile coords (~ dx)
 * - p1 = (bit 24-25) - transport type.
 * @param p2 various bitstuffed elements
 * - p2 = (bit  0- 7) - bridge type (hi bh)
 * - p2 = (bit  8-13) - rail type or road types.
 * - p2 = (bit 16-31) - town index
 * @param text unused
 * @return the cost of this operation or an error
 */