flowgrind: drop dependency to fg_socket.h

The flogwrind controller should not depend on fg_socket and daemon.h.
Drop that dependency.
flowgrindd: separate out init of rpc server

This separates initialization of rpc server from running it.
Initialization is now done before flowgrind forks into the background,
therefore it can fail fast and rpc related error early. Closes #154
flowgrindd: add some rpc server related doxygen comments
Clarify comment on netinet/tcp.h
Merge pull request #158 from arnd/rpcrefactor

Rpcrefactor