A couple of suggestions:

- there's no reason to require autoreallocation in buffers; if a buffer
  can't be reallocated and we're trying to reserve too much space, then
  isc_buffer_reserve() just returns ISC_R_NOSPACE.
- we can work out the number of characters added to a buffer in other
  ways, but it's useful to float result codes up the call stack, so
  let's have isc_buffer_printf() return isc_result_t.

I've pushed a commit that makes these changes - what do you think?