This CL adds the Utimensat syscall and AT_SYMLINK_NOFOLLOW constant.

This is required for setting the access/modification times on symlinks.

In addition, it updates the UtimesNano function to pass 0 as the flags
parameter, to avoid potentially passing junk.

Change-Id: I280645f3f53173628b1e1986bc7a47bac254fcf8
Reviewed-on: https://go-review.googlesource.com/9379Reviewed-by: Rob Pike <r@golang.org>