Refactor node ID handling, use check digits (fixes #269)

New node ID:s contain four Luhn check digits and are grouped
differently. Code uses NodeID type instead of string, so it's formatted
homogenously everywhere.

cmd/stcli/tls.go

@@ -5,20 +5,12 @@
 package main
 
 import (
-	"crypto/sha256"
 	"crypto/tls"
-	"encoding/base32"
 	"path/filepath"
-	"strings"
 )
 
 func loadCert(dir string) (tls.Certificate, error) {
-	return tls.LoadX509KeyPair(filepath.Join(dir, "cert.pem"), filepath.Join(dir, "key.pem"))
-}
-
-func certID(bs []byte) string {
-	hf := sha256.New()
-	hf.Write(bs)
-	id := hf.Sum(nil)
-	return strings.Trim(base32.StdEncoding.EncodeToString(id), "=")
+	cf := filepath.Join(dir, "cert.pem")
+	kf := filepath.Join(dir, "key.pem")
+	return tls.LoadX509KeyPair(cf, kf)
 }