-info: add contentEncryption

Example:

$ ./gocryptfs -info ./tests/example_filesystems/v2.2-xchacha/
Creator:           gocryptfs v2.1-27-gabaa129-dirty.xchacha
FeatureFlags:      HKDF XChaCha20Poly1305 DirIV EMENames LongNames Raw64
EncryptedKey:      64B
ScryptObject:      Salt=32B N=1024 R=8 P=1 KeyLen=32
contentEncryption: XChaCha20-Poly1305

info.go

@@ -1,44 +1,30 @@
 package main
 
 import (
-	"encoding/json"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"strings"
 
 	"github.com/rfjakob/gocryptfs/v2/internal/configfile"
-	"github.com/rfjakob/gocryptfs/v2/internal/contentenc"
 	"github.com/rfjakob/gocryptfs/v2/internal/exitcodes"
-	"github.com/rfjakob/gocryptfs/v2/internal/tlog"
 )
 
 // info pretty-prints the contents of the config file at "filename" for human
 // consumption, stripping out sensitive data.
 // This is called when you pass the "-info" option.
 func info(filename string) {
-	// Read from disk
-	js, err := ioutil.ReadFile(filename)
+	cf, err := configfile.Load(filename)
 	if err != nil {
-		tlog.Fatal.Printf("Reading config file failed: %v", err)
-		os.Exit(exitcodes.LoadConf)
-	}
-	// Unmarshal
-	var cf configfile.ConfFile
-	err = json.Unmarshal(js, &cf)
-	if err != nil {
-		tlog.Fatal.Printf("Failed to unmarshal config file")
-		os.Exit(exitcodes.LoadConf)
-	}
-	if cf.Version != contentenc.CurrentVersion {
-		tlog.Fatal.Printf("Unsupported on-disk format %d", cf.Version)
+		fmt.Printf("Loading config file failed: %v\n", err)
 		os.Exit(exitcodes.LoadConf)
 	}
+	s := cf.ScryptObject
+	algo, _ := cf.ContentEncryption()
 	// Pretty-print
 	fmt.Printf("Creator:           %s\n", cf.Creator)
 	fmt.Printf("FeatureFlags:      %s\n", strings.Join(cf.FeatureFlags, " "))
 	fmt.Printf("EncryptedKey:      %dB\n", len(cf.EncryptedKey))
-	s := cf.ScryptObject
 	fmt.Printf("ScryptObject:      Salt=%dB N=%d R=%d P=%d KeyLen=%d\n",
 		len(s.Salt), s.N, s.R, s.P, s.KeyLen)
+	fmt.Printf("contentEncryption: %s\n", algo.Algo) // lowercase because not in JSON
 }