This makes evprc setup more extensible, and helps with Shuo Chen's
work on implementing Google protocol buffers rpc on top of Libevent 2
evrpc.
This patch breaks binary compatibility with previous versions of
Libevent, since it changes struct evrpc and the signature of
evrpc_register_generic(). Since all compliant code should be calling
evrpc_register_generic via EVRPC_REGISTER, it shouldn't break source
compatibility.
(Code by Shuo Chen; commit message by Nick)
