easier dock placement patch

[xarick]

Purpose of this patch is to ease the rules a bit when placing docks, in regards to the third tile check, the water tile.

v1:
- It allows construction if it is an aqueduct head.
- It allows construction if it is a buoy.
- It allows construction if it is a water tile with one corner raised and without a rail track on the opposite corner.

v2:
- It allows construction if it is a water tile with one corner raised and with a rail track on the opposite corner.

v3:
- Changed the way the code checks for stuff, so that it doesn't allow the docks to be placed in front of other docks, as well as oil rigs and probably some other tile types that I'm not even aware which would end in the dock entrance being innaccessible for ships (due to noob coding skills).

Note: There is a similar patch here viewtopic.php?f=33&t=75230 but without allowing construction on water tiles with one corner raised, for AI compatibility purposes.