envknob: generalize Windows tailscaled-env.txt support

ipnserver previously had support for a Windows-only environment
variable mechanism that further only worked when Windows was running
as a service, not from a console.

But we want it to work from tailscaed too, and we want it to work on
macOS and Synology. So move it to envknob, now that envknob can change
values at runtime post-init.

A future change will wire this up for more platforms, and do something
more for CLI flags like --port, which the bug was originally about.

Updates #5114

Change-Id: I9fd69a9a91bb0f308fc264d4a6c33e0cbe352d71
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server.go

@@ -932,14 +932,6 @@ func BabysitProc(ctx context.Context, args []string, logf logger.Logf) {
 		startTime := time.Now()
 		log.Printf("exec: %#v %v", executable, args)
 		cmd := exec.Command(executable, args...)
-		if runtime.GOOS == "windows" {
-			extraEnv, err := loadExtraEnv()
-			if err != nil {
-				logf("errors loading extra env file; ignoring: %v", err)
-			} else {
-				cmd.Env = append(os.Environ(), extraEnv...)
-			}
-		}
 
 		// Create a pipe object to use as the subproc's stdin.
 		// When the writer goes away, the reader gets EOF.
@@ -1208,38 +1200,3 @@ func findQnapTaildropDir(name string) (string, error) {
 	}
 	return "", fmt.Errorf("shared folder %q not found", name)
 }
-
-func loadExtraEnv() (env []string, err error) {
-	if runtime.GOOS != "windows" {
-		return nil, nil
-	}
-	name := filepath.Join(os.Getenv("ProgramData"), "Tailscale", "tailscaled-env.txt")
-	contents, err := os.ReadFile(name)
-	if os.IsNotExist(err) {
-		return nil, nil
-	}
-	if err != nil {
-		return nil, err
-	}
-	for _, line := range strings.Split(string(contents), "\n") {
-		line = strings.TrimSpace(line)
-		if line == "" || line[0] == '#' {
-			continue
-		}
-		k, v, ok := strings.Cut(line, "=")
-		if !ok || k == "" {
-			continue
-		}
-		if strings.HasPrefix(v, `"`) {
-			var err error
-			v, err = strconv.Unquote(v)
-			if err != nil {
-				return nil, fmt.Errorf("invalid value in line %q: %v", line, err)
-			}
-			env = append(env, k+"="+v)
-		} else {
-			env = append(env, line)
-		}
-	}
-	return env, nil
-}