Correct logic for realigning a chain in evbuffer_add

Correct logic for realigning a chain in evbuffer_add

The old logic was both too eager to realign (it would move a whole
chain to save a byte) and too reluctant to realign (it would only
realign when data would fit into the misaligned portion, without
considering the space at the end of the chain).

The new logic matches that from evbuffer_expand_singlechain: it only
realigns a chain when not much data is to be moved, and there's a
bunch of space to be regained.

Spotted by Yan Lin.