Permit co-entry on 4-way tracks with no conflict

[poppo]

I make a patch4way_update.patch which allow 2 convoys to enter the 4-way tile only if they do not "conflict" each other.


When two convoys are conflict (e.g. N->S and E->W), they cannot enter(reserve) in the same time (see the convoys in the right side of this figure).


If you add paks, you can reduce the spacing between two digaonal tracks.


Technically, I add
  ribi_t::ribi reserved_dir
  convoihandle_t reserved_2
in schiene_t to check the convoys conflicts in 4-way tracks, and update how to resere/unreserve tracks.