unix: add Fstatat on DragonflyBSD

Change-Id: Ie4a172a36a274334a49766cc272e74bcc34ebd86
Reviewed-on: https://go-review.googlesource.com/100576
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_dragonfly.go

@@ -255,6 +255,7 @@ func Uname(uname *Utsname) error {
 //sys	Flock(fd int, how int) (err error)
 //sys	Fpathconf(fd int, name int) (val int, err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
+//sys	Fstatat(fd int, path string, stat *Stat_t, flags int) (err error)
 //sys	Fstatfs(fd int, stat *Statfs_t) (err error)
 //sys	Fsync(fd int) (err error)
 //sys	Ftruncate(fd int, length int64) (err error)

unix/zsyscall_dragonfly_amd64.go

@@ -659,6 +659,21 @@ func Fstat(fd int, stat *Stat_t) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func Fstatat(fd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_FSTATAT, uintptr(fd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Fstatfs(fd int, stat *Statfs_t) (err error) {
 	_, _, e1 := Syscall(SYS_FSTATFS, uintptr(fd), uintptr(unsafe.Pointer(stat)), 0)
 	if e1 != 0 {