types/result, util/lineiter: add package for a result type, use it

This adds a new generic result type (motivated by golang/go#70084) to
try it out, and uses it in the new lineutil package (replacing the old
lineread package), changing that package to return iterators:
sometimes over []byte (when the input is all in memory), but sometimes
iterators over results of []byte, if errors might happen at runtime.

Updates #12912
Updates golang/go#70084

Change-Id: Iacdc1070e661b5fb163907b1e8b07ac7d51d3f83
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/pidowner/pidowner_linux.go

@@ -8,26 +8,26 @@ import (
 	"os"
 	"strings"
 
-	"tailscale.com/util/lineread"
+	"tailscale.com/util/lineiter"
 )
 
 func ownerOfPID(pid int) (userID string, err error) {
 	file := fmt.Sprintf("/proc/%d/status", pid)
-	err = lineread.File(file, func(line []byte) error {
+	for lr := range lineiter.File(file) {
+		line, err := lr.Value()
+		if err != nil {
+			if os.IsNotExist(err) {
+				return "", ErrProcessNotFound
+			}
+			return "", err
+		}
 		if len(line) < 4 || string(line[:4]) != "Uid:" {
-			return nil
+			continue
 		}
 		f := strings.Fields(string(line))
 		if len(f) >= 2 {
 			userID = f[1] // real userid
 		}
-		return nil
-	})
-	if os.IsNotExist(err) {
-		return "", ErrProcessNotFound
-	}
-	if err != nil {
-		return
 	}
 	if userID == "" {
 		return "", fmt.Errorf("missing Uid line in %s", file)