fdreserve_locked
static void fdreserve_locked (struct filedesc *fdp, int fd0, int incr);
fdreserve_locked(fdp, new, 1);
fdreserve_locked(fdp, fd, 1);
fdreserve_locked(fdp, fd, -1);
fdreserve_locked(fdp, fd, -1);
fdreserve_locked(newfdp, i, -1);