net/dns: fall back to copy+delete/truncate if moving to/from /etc/resolv.conf fails.

In some containers, /etc/resolv.conf is a bind-mount from outside the container.
This prevents renaming to or from /etc/resolv.conf, because it's on a different
filesystem from linux's perspective. It also prevents removing /etc/resolv.conf,
because doing so would break the bind-mount.

If we find ourselves within this environment, fall back to using copy+delete when
renaming to /etc/resolv.conf, and copy+truncate when renaming from /etc/resolv.conf.

Fixes #3000

Co-authored-by: Denton Gentry <dgentry@tailscale.com>
Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/manager_linux_test.go

@@ -186,8 +186,20 @@ func (m memFS) ReadFile(name string) ([]byte, error) {
 	panic("TODO")
 }
 
-func (fs memFS) WriteFile(name string, contents []byte, perm os.FileMode) error {
-	fs[name] = string(contents)
+func (m memFS) Truncate(name string) error {
+	v, ok := m[name]
+	if !ok {
+		return fs.ErrNotExist
+	}
+	if s, ok := v.(string); ok {
+		m[name] = s[:0]
+	}
+
+	return nil
+}
+
+func (m memFS) WriteFile(name string, contents []byte, perm os.FileMode) error {
+	m[name] = string(contents)
 	return nil
 }