fragmentation in evbuffer_expand by increasing the size of the last buffer
in the chain; as a result with have to keep track of the previous_to_last
chain; provide a evbuffer_validate() function in the regression test to
make sure that all evbuffer are internally consistent.
svn:r699
Make offsetof into evutil_offsetof. Be a little more willing to call evbuffer_chain_align() from evbuffer_expand(). Clarify some docs, and add some XXX comments to note questionable areas.
svn:r677
