unix: use unsafe.Slice instead of unsafeheader package

Go 1.18 is the minimum supported Go version and unsafe.Slice was
introduced in Go 1.17.

Change-Id: Ie5f1fad01f219e7b7a190de2c49676f366ad3bc7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/428515
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_darwin.1_13.go

@@ -7,11 +7,7 @@
 
 package unix
 
-import (
-	"unsafe"
-
-	"golang.org/x/sys/internal/unsafeheader"
-)
+import "unsafe"
 
 //sys	closedir(dir uintptr) (err error)
 //sys	readdir_r(dir uintptr, entry *Dirent, result **Dirent) (res Errno)
@@ -86,11 +82,7 @@ func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 		}
 
 		// Copy entry into return buffer.
-		var s []byte
-		hdr := (*unsafeheader.Slice)(unsafe.Pointer(&s))
-		hdr.Data = unsafe.Pointer(&entry)
-		hdr.Cap = reclen
-		hdr.Len = reclen
+		s := unsafe.Slice((*byte)(unsafe.Pointer(&entry)), reclen)
 		copy(buf, s)
 
 		buf = buf[reclen:]