unix: add Openat on dragonfly

Change-Id: I1bf134301e5e81e6d6524bf63532975166694db6
Reviewed-on: https://go-review.googlesource.com/c/144978
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

@@ -287,6 +287,7 @@ func Uname(uname *Utsname) error {
 //sys	Mknod(path string, mode uint32, dev int) (err error)
 //sys	Nanosleep(time *Timespec, leftover *Timespec) (err error)
 //sys	Open(path string, mode int, perm uint32) (fd int, err error)
+//sys	Openat(dirfd int, path string, mode int, perm uint32) (fd int, err error)
 //sys	Pathconf(path string, name int) (val int, err error)
 //sys	read(fd int, p []byte) (n int, err error)
 //sys	Readlink(path string, buf []byte) (n int, err error)

unix/zsyscall_dragonfly_amd64.go

@@ -1023,6 +1023,22 @@ func Open(path string, mode int, perm uint32) (fd int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func Openat(dirfd int, path string, mode int, perm uint32) (fd int, err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	r0, _, e1 := Syscall6(SYS_OPENAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(mode), uintptr(perm), 0, 0)
+	fd = int(r0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Pathconf(path string, name int) (val int, err error) {
 	var _p0 *byte
 	_p0, err = BytePtrFromString(path)