net/dns: rename resolvconf.go to debian_resolvconf.go.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/manager_freebsd.go

@@ -9,12 +9,7 @@ import "tailscale.com/types/logger"
 func NewOSConfigurator(logf logger.Logf, _ string) OSConfigurator {
 	switch {
 	case isResolvconfActive():
-		if resolvconfIsOpenresolv() {
-			return newOpenresolvManager()
-		} else {
-			// Debian resolvconf
-			return newResolvconfManager(logf)
-		}
+		return newResolvconfManager(logf)
 	default:
 		return newDirectManager()
 	}