/**
* 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
*/