This is a bit of an interface doozy, but it's really needed in order
to be able to document this stuff without apologizing it. This patch
does the following renamings:
evbuffercb -> bufferevent_data_cb
everrorcb -> bufferevent_event_cb
EVBUFFER_(READ,WRITE,...) -> BEV_EVENT_(...)
EVBUFFER_(INPUT,OUTPUT) -> bufferevent_get_(input,output)
All the old names are available in event2/bufferevent_compat.h
svn:r1283
