os: implement readdir for darwin and linux

readdir is disabled on linux for 386 and arm until syscall.seek is implemented there.

windows is hard, so leaving that for later.

File src/os/dir_other_go115.go can be deleted when we drop support for go 1.15.

Also adds TestReadNonDir, which was helpful while debugging.

src/os/dirent_linux.go

@@ -0,0 +1,53 @@
+// +build !baremetal,!js,!wasi
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func direntIno(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(syscall.Dirent{}.Ino), unsafe.Sizeof(syscall.Dirent{}.Ino))
+}
+
+func direntReclen(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(syscall.Dirent{}.Reclen), unsafe.Sizeof(syscall.Dirent{}.Reclen))
+}
+
+func direntNamlen(buf []byte) (uint64, bool) {
+	reclen, ok := direntReclen(buf)
+	if !ok {
+		return 0, false
+	}
+	return reclen - uint64(unsafe.Offsetof(syscall.Dirent{}.Name)), true
+}
+
+func direntType(buf []byte) FileMode {
+	off := unsafe.Offsetof(syscall.Dirent{}.Type)
+	if off >= uintptr(len(buf)) {
+		return ^FileMode(0) // unknown
+	}
+	typ := buf[off]
+	switch typ {
+	case syscall.DT_BLK:
+		return ModeDevice
+	case syscall.DT_CHR:
+		return ModeDevice | ModeCharDevice
+	case syscall.DT_DIR:
+		return ModeDir
+	case syscall.DT_FIFO:
+		return ModeNamedPipe
+	case syscall.DT_LNK:
+		return ModeSymlink
+	case syscall.DT_REG:
+		return 0
+	case syscall.DT_SOCK:
+		return ModeSocket
+	}
+	return ^FileMode(0) // unknown
+}