unix: don't use syscall.ParseDirent

Implement ParseDirent in x/sys/unix instead of calling
syscall.ParseDirent. The latter uses offsets into syscall.Dirent which
might not be matching unix.Dirent depending on Go version. This is e.g.
the case with of FreeBSD whose Dirent structure was updated for Go 1.12.

This fixes TestDirent and TestGetdirentries on freebsd with Go 1.11

Reverts CL 88475

Change-Id: I04318f59c6fbf148c75ce3667255a0c0428288e2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/183897
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

unix/syscall_dragonfly.go

@@ -57,6 +57,22 @@ func nametomib(name string) (mib []_C_int, err error) {
 	return buf[0 : n/siz], nil
 }
 
+func direntIno(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(Dirent{}.Fileno), unsafe.Sizeof(Dirent{}.Fileno))
+}
+
+func direntReclen(buf []byte) (uint64, bool) {
+	namlen, ok := direntNamlen(buf)
+	if !ok {
+		return 0, false
+	}
+	return (16 + namlen + 1 + 7) &^ 7, true
+}
+
+func direntNamlen(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(Dirent{}.Namlen), unsafe.Sizeof(Dirent{}.Namlen))
+}
+
 //sysnb pipe() (r int, w int, err error)
 
 func Pipe(p []int) (err error) {