Replace -worldpath with -worldselectpath

This reverts commit 7ad46abced. This
reintroduces the worldpath argument as per request from #1914, but at a
different name. This is because users have configurations like this,
which no longer work:

-world + -worldpath = crash

If you want to use -worldselectionpath to specify a world, you should be
able to use -worldname, but don't use -world unless you specify an
absolute path to a world.

No matter how we solve this we get a support headache (-worldpath +
-world = crash). This temporary stopgap should work to help address
issue #1914 until we can figure out a final solution. Since users are
impacted by this change, temporarily adding this back is the best move.

To be 100% clear, though:

-world + -worldselectpath without specifying an absolute path will
result in a crash that is unhelpful. Please don't do that.

TShockAPI/TShock.cs

@@ -689,6 +689,16 @@ namespace TShockAPI
 						}
 					})
 
+				.AddFlag("-worldselectpath", pathChecker)
+					.After(() =>
+					{
+						if (path != null)
+						{
+							Main.WorldPath = path;
+							ServerApi.LogWriter.PluginWriteLine(this, "World path has been set to " + path, TraceLevel.Info);
+						}
+					})
+
 				.AddFlag("-logpath", pathChecker)
 					.After(() =>
 					{