connection support via R_ext/Connection.h

[tomoakin]

In recent version of R (3.3.1), there is R_ext/Connection.h but no R_ext/RConn.h.
This is somewhat from the situation for "RConn patch", but connection is integrated into the main R source. It should be worthwhile to make them work with Cairo again.
By just matching the types, the code seems to work, though I have not tested very thoroughly yet and there seems to room for cleaner codes. Nonetheless, I would show you this code as this could be some starting point to support again.

[s-u]

Thanks for the contribution! Unfortunately, some other CRAN members still don't consider the connection API official enough to be used on CRAN, so I won't merge this until it is allowed on CRAN. However, it is certainly very useful and merges cleanly so anyone who wants this functionality can use it.