1. I feel this is a bikeshed issue
2. In the spirit of bikeshedding, I don't see how calling it 'buffer' is going to help understand it - you're still going to have to read a handful of lines of (rather trivial) code. There's nothing clever happening in this code.
3. Calling it 'buffer' will suggest it is a field along with 'buf' and 'buflen', but it is not.
4. To my mind, calling it 'buffer' suggests it is more than a mere pointer, as in D one tends to think of a buffer as being an array or other more complex type.