Add the functions that changed between BTL 2.0 and 3.0 into compat.h
and compat.c:
* module.btl_prepare_src: the signature and body of this method
changed between 2.0 and 3.0. However, the functions that this
method calls did *not* need to change, so they are copied over
wholesale (with the exception that they no longer accept the unused
`registration` parameter).
* module.btl_prepare_dst: this method does not exist in BTL 3.0.
* module.btl_put: the signature and body of this method changed
between 2.0 and 3.0.
67ee1e6cf8