repl: fix persistent history and env variable name

wasn't able to get truthy, which in turn broke persistent history
completely. This changes the variable to get truthy on true terminals.

Additionally, the docs and the code did differ on which environment
variable was used for history. This changes the code to use
NODE_REPL_HISTORY_FILE.

lib/internal/repl.js

@@ -26,8 +26,9 @@ module.paths = require('module')._nodeModulePaths(module.filename);
 
 function createRepl(env, cb) {
   const opts = {
-    useGlobal: true,
-    ignoreUndefined: false
+    ignoreUndefined: false,
+    terminal: process.stdout.isTTY,
+    useGlobal: true
   };
 
   if (parseInt(env.NODE_NO_READLINE)) {
@@ -57,8 +58,8 @@ function createRepl(env, cb) {
   }
 
   const repl = REPL.start(opts);
-  if (opts.terminal && env.NODE_REPL_HISTORY_PATH) {
-    return setupHistory(repl, env.NODE_REPL_HISTORY_PATH, cb);
+  if (opts.terminal && env.NODE_REPL_HISTORY_FILE) {
+    return setupHistory(repl, env.NODE_REPL_HISTORY_FILE, cb);
   }
   repl._historyPrev = _replHistoryMessage;
   cb(null, repl);
@@ -158,7 +159,7 @@ function _replHistoryMessage() {
   if (this.history.length === 0) {
     this._writeToOutput(
         '\nPersistent history support disabled. ' +
-        'Set the NODE_REPL_HISTORY_PATH environment variable to ' +
+        'Set the NODE_REPL_HISTORY_FILE environment variable to ' +
         'a valid, user-writable path to enable.\n'
     );
     this._refreshLine();