added ares_process_fd() to allow applications to ask for processing on

added ares_process_fd() to allow applications to ask for processing on
specific sockets and thus avoiding select() and associated functions/macros.
This function will be used by upcoming libcurl releases for this very
reason. It also made me export the ares_socket_t type in the public ares.h
header file, since ares_process_fd() uses that type for two of the arguments.