Add -forcec option to force C calling convention for D functions.

[alexrp]

As discussed on the NG.

I'm not entirely sure whether this covers all cases, but through grepping the source, this seems to be the place to add it.

[deadalnix]

What about explicit extern(D) ? This is an open question.

[alexrp]

As intended, that will use D name mangling, but use C calling convention when you pass -forcec.

[ghost]

But how are you supposed to link apps with libraries that are compiled this way? And especially multiple libraries where one was compiled with -forcec and the other was not.