* And rename accordingly.

* This simplifies effectively disabling this feature.

* Because inter-communicators cannot be used with the neighbor alltoall
  exchanger, an alternative needs to be picked in that case.

* This will enable saving MPI_Type_dup calls later on, where
  ownership can be simply transferred.

* This way, the constructor cannot ever be invoked if the MPI version is < 3.

* Setting XT_CONFIG_DEFAULT_EXCHANGE_METHOD allows for an adjusted default.

* This facilitates per-call selection of the underlying exchanger.

* Also fix missing/malformed license headers and conditional inclusion macros
  where found.