Clarify functions and add additional utilities.

Some of the callback functions specified by the messager struct were
badly named; their names have been updated to reflect their actual
usage.

Additionally, the GLib implementations provide some utility functions
that make looping really easy. A complete event loop using poll(2) now
looks something like this:

    while (!curvecpr_client_messager_glib_is_finished(cmg)) {
        long long timeout =
            curvecpr_client_messager_glib_next_timeout(cmg) / 1000000;

        struct pollfd polls[1] = {
            { .fd = my_socket, .events = POLLIN }
        };
        if (poll(polls, 1, timeout) < 0)
            break;

        if (polls[0].revents) {
            unsigned char buf[1152];
            int r = recvfrom(my_socket, buf, 1152, 0, NULL, NULL);
            if (r <= 0)
                break;

            curvecpr_client_messager_glib_recv(cmg, buf, r);
        }

        curvecpr_client_messager_glib_process_sendq(cmg);
    }