Add DisableInterRegionTeleportCancellation option in [EntityTransfer] section of OpenSim.ini.  False by default.

This option allows the simulator to specify that the cancel button on inter-region teleports should never appear.
This exists because sometimes cancellation will result in a stuck avatar requiring relog.
It may be hard to prevent this due to the protocol design (the LL grid has the same issue)
In small controlled grids where teleport failure is practically impossible it can be better to disable teleport cancellation entirely.