Enable three-way turnouts with proper conns support on all sides

I chose to make three-way turnouts have 5 conns (last one is not used) so
that they can be distinguished from crossings easily without refactoring the
code. Three-ways should have their last entry with {["3"]=0} instead as a sort
of internal mark.

advtrains/helpers.lua

@@ -334,9 +334,10 @@ function advtrains.get_adjacent_rail(this_posnr, this_conns_p, conn_idx, drives_
 end
 
 -- when a train enters a rail on connid 'conn', which connid will it go out?
--- nconns: number of connections in connection table
+-- nconns: number of connections in connection table:
+-- 2 = straight rail; 3 = turnout, 4 = crossing, 5 = three-way turnout (5th entry is a stub)
 -- returns: connid_out
-local connlku={[2]={2,1}, [3]={2,1,1}, [4]={2,1,4,3}}
+local connlku={[2]={2,1}, [3]={2,1,1}, [4]={2,1,4,3}, [5]={2,1,1,1}}
 function advtrains.get_matching_conn(conn, nconns)
 	return connlku[nconns][conn]
 end

advtrains/tracks.lua

@@ -124,8 +124,7 @@ advtrains.ap.t_s3way={
 	regstep=1,
 	variant={
 		l={
-			--conns = conns6(0,7,0,8,0,9),
+			conns = { {c=0}, {c=7}, {c=8}, {c=9}, {["3"]=0} },
-			conns = conns3(0,7,8),
 			desc = "3-way turnout (left)",
 			switchalt = "s",
 			--switchalt2 = ,
@@ -133,16 +132,14 @@ advtrains.ap.t_s3way={
 			switchst="l",
 		},
 		s={
-			--conns = conns6(0,8,0,7,0,9),
+			conns = { {c=0}, {c=8}, {c=7}, {c=9}, {["3"]=0} },
-			conns = conns3(0,8,7),
 			desc = "3-way turnout (straight)",
 			switchalt ="r",
 			--switchmc = "on",
 			switchst = "s",
 		},
 		r={
-			--conns = conns6(0,9,0,8,0,7),
+			conns = { {c=0}, {c=9}, {c=8}, {c=7}, {["3"]=0} },
-			conns = conns3(0,9,8),
 			desc = "3-way turnout (right)",
 			switchalt = "l",
 			switchst="r"