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_openbsd.go

@@ -6,6 +6,6 @@ package dns
 
 import "tailscale.com/types/logger"
 
-func NewOSConfigurator(logger.Logf, string) (OSConfigurator, error) {
-	return newDirectManager(), nil
+func NewOSConfigurator(logf logger.Logf, _ string) (OSConfigurator, error) {
+	return newDirectManager(logf), nil
 }