Don't store RAILWAYS track in sectors

Don't store RAILWAYS track in sectors

Storing track in sectors is problematic, because we need to update
adjacent sectors when updating a sector in a way that changes its
capability to extend railway into its neighbors.  This invalidates
cached adjacent sectors, and calling code may not be prepared for
that.  Specifically, bridge building caches the bridge head, and
writes it back later, wiping out the track update.

Replace struct sctstr member sct_track by new sct_rail_track().  Make
selector track virtual.  Remove the code to keep sct_track up-to-date:
set_railway(), update_railway().

Unfortunately, this causes cyclic dependencies between link libraries:
the virtual selector needs to be referenced from src/lib/global/nsc.c,
and it needs to reference stuff from src/lib/common/file.c.  Hack
around it in Make.mk for now.