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>

hostinfo/hostinfo_linux.go

@@ -12,7 +12,7 @@ import (
 
 	"golang.org/x/sys/unix"
 	"tailscale.com/types/ptr"
-	"tailscale.com/util/lineread"
+	"tailscale.com/util/lineiter"
 	"tailscale.com/version/distro"
 )
 
@@ -106,15 +106,18 @@ func linuxVersionMeta() (meta versionMeta) {
 	}
 
 	m := map[string]string{}
-	lineread.File(propFile, func(line []byte) error {
+	for lr := range lineiter.File(propFile) {
+		line, err := lr.Value()
+		if err != nil {
+			break
+		}
 		eq := bytes.IndexByte(line, '=')
 		if eq == -1 {
-			return nil
+			continue
 		}
 		k, v := string(line[:eq]), strings.Trim(string(line[eq+1:]), `"'`)
 		m[k] = v
-		return nil
-	})
+	}
 
 	if v := m["VERSION_CODENAME"]; v != "" {
 		meta.DistroCodeName = v