Add various methods and flags to Syscall

* Add OpenFlags.O_CLOEXEC and OpenFlags.O_PATH

* Add AtFlags for AT_* values

* Add fdopendir, mkdtemp, futimens

* Add readv, writev, preadv, pwritev

* Add *at methods: openat, renameat, fchmodat, fstatat, utimensat,
      mkdirat, mknodat, mkfifoat, faccessat, fchownat, linkat, readlinkat,
      symlinkat, unlinkat

* Add constants AT_FDCWD, UTIME_NOW, UTIME_OMIT

* Syscall.readdir(): Set errno to 0 before calling readdir to allow the
      caller to distinguish between errors and end-of-directory.