New option RAILWAYS

New option RAILWAYS

With RAILWAYS, highway-like sectors double as rail.  They need to be
at least 5% efficient to be operational, and then they additionally
extend rail into adjacent sectors that are at least 60% efficient.

New opt_RAILWAYS, SCT_HAS_RAIL(), sct_rail_track().  Update
sector_mcost(), bp_neighbors(), lnd_mar_one_sector() for RAILWAYS
mobility rules.  Update sinfra(), spyline(), satdisp_sect() to show
rail track instead of rail infrastructure for RAILWAYS.

New virtual sector selector track, implemented by nsc_sct_track().