(sector_mcost): New, simpler sector mobility cost formula. The cost

(sector_mcost): New, simpler sector mobility cost formula.  The cost
is a linear function of sector efficiency, with cost at 0% given by
dchstr member d_mcst and cost at 100% given by d_emcst.  The latter
used to be d_mcst - 1.  Mountain cost is no longer a special case.
Road infrastructure now reduces cost up to 90%, linear in road
efficiency (used to be close to that, but non-linear).  Same for rail
infrastructure, but 99%.  Double land unit minimum cost to 0.02.
Change land unit cost penalty in newly taken land to an extra 0.2
instead of a minimum of 0.2.
(dchrstr): New member d_emcst.  sector.config updated, with some
changes: cost of mountain now ranges from 2.4 to 1.2 instead of 2.174
to 1.304, wildernes 0.4 instead of 0.6 to 0.4, highway, bridge head,
bridge span and tower from 0.4 to 0.001 instead of 0.2 to 0.001.
While I'm at it, make bridge head match highway stats (dstr and value
reduced).
(show_sect_stats): Show d_emcst.
(dchr_ca): New selector emcst for d_emcst.