gctl_change_param
error = gctl_change_param(req, "iterations", sizeof(intmax_t),
gctl_change_param(req, "size", -1, ssize);
error = gctl_change_param(req, GPART_PARAM_BOOTCODE, bootsize,
gctl_change_param(req, "size", -1, ssize);
gctl_change_param(req, "size", -1, ssize);
gctl_change_param(req, "start", -1, sstart);
int gctl_change_param(struct gctl_req *req, const char *name, int len,