msxml3: get_attributes not supported for Attribute, CData, Comment, Doc, Doc Frag, Entity and Text Nodes.

First of all, the code was freeing the wrong pointer (i.e. the pointer
supplied by the caller of one of the NdrFullPointer* functions, not the
PFULL_PTR_TO_REFID_ELEMENT. Second, the code wasn't following the Next
link to the next entry in the list.

Add tests for this function.

The actual sending of the request is still done in I_RpcSendReceive though.

Disallow the server from calling I_RpcSend and I_RpcReceive to allow 
simplification of the code. The releasing of the connection is now done 
in I_RpcFreeBuffer.

Implement I_RpcNegotiateTransferSyntax.

Only call IRpcChannelBuffer_FreeBuffer in NdrProxyFreeBuffer if 
fBufferValid is set.