Adjust signal on bridge/tunnel map bits, rename accessors, add docs.

Use same bit to store red/green state for entrances and exits.
No longer re-use exit bit to store red/green state of entrance.
Avoid modifying the M2 of tunnel entrances/exits.
Rename and rationalise map accessor functions.
Document bits used in landscape HTML docs.

docs/landscape.html

@@ -1446,7 +1446,7 @@
      <li>m3 bits 7..4: <a href="#OwnershipInfo">owner</a> of tram</li>
      <li>m3 bits 3..0: <a href="#TrackType">track type</a> for railway</li>
      <li>m5 bit 4: pbs reservation state for railway</li>
-     <li>m5 bits 7 clear: tunnel entrance/exit</li>
+     <li>m5 bit 7 clear: tunnel entrance/exit</li>
      <li>m5 bit 7 set: bridge ramp
       <ul>
        <li>m6 bits 5..2: <a name="BridgeType">bridge type</a>:
@@ -1538,6 +1538,31 @@
        </li>
       </ul>
      </li>
+     <li style="color: blue">m5 bits 6..5: signal simulation (signals on bridge/tunnels patch)
+      <table style="color: blue">
+       <tr>
+        <td><tt>0</tt>&nbsp;</td>
+        <td>no signal simulation</td>
+       </tr>
+
+       <tr>
+        <td><tt>1</tt>&nbsp;</td>
+        <td>signal simulation entrance (bit 5 set)</td>
+       </tr>
+
+       <tr>
+        <td><tt>2</tt>&nbsp;</td>
+        <td>signal simulation exit (bit 6 set)</td>
+       </tr>
+      </table>
+      If signal simulation entrance or exit:
+      <ul>
+       <li>m6 bit 6: set = PBS signals, clear = block signals</li>
+       <li>m6 bit 1: set = semaphore signals, clear = light signals</li>
+       <li>m6 bit 0: set = signal shows green, clear = signal shows red</li>
+       <li>m2 bits 15..0: for bridges only: for signals 0..15 on bridge, signal is visually red if corresponding bit in 0..15 is set</li>
+      </ul>
+     </li>
      <li>m5 bits 3..2: transport type
       <table>
        <tr>