unix: add Fchmodat on DragonflyBSD

Change-Id: I3fd96550d588373d75e14bd554d46e2590c266fb
Reviewed-on: https://go-review.googlesource.com/101596
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_dragonfly.go

@@ -251,6 +251,7 @@ func Uname(uname *Utsname) error {
 //sys	Fchdir(fd int) (err error)
 //sys	Fchflags(fd int, flags int) (err error)
 //sys	Fchmod(fd int, mode uint32) (err error)
+//sys	Fchmodat(dirfd int, path string, mode uint32, flags int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Flock(fd int, how int) (err error)
 //sys	Fpathconf(fd int, name int) (val int, err error)

unix/zsyscall_dragonfly_amd64.go

@@ -618,6 +618,21 @@ func Fchmod(fd int, mode uint32) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_FCHMODAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(mode), 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 Fchown(fd int, uid int, gid int) (err error) {
 	_, _, e1 := Syscall(SYS_FCHOWN, uintptr(fd), uintptr(uid), uintptr(gid))
 	if e1 != 0 {