varlink: move O_NONBLOCK setting to varlink_server_listen_fd(), and set O_CLOEXEC too

varlink: move O_NONBLOCK setting to varlink_server_listen_fd(), and set O_CLOEXEC too

Let's move setting of O_NONBLOCK into varlink_server_listen_fd() and out
of varlink_server_create_listen_fd_socket(). The latter has two callers:
varlink_server_listen_fd() and varlink_server_listen_address(), which
guarantees O_CLOEXEC+O_NONBLOCK anyway, hence no neet to repeat the
logic.