Also make DownloadManager NAM private if private browsing is on

There's only one global DownloadManager with its own NAM (for downloads not
associated with a page). We can't really decide whether that should be private
or not, so as a best-effort approximation we simply make it private if private
browsing was turned on when starting qutebrowser.

qutebrowser/browser/qtnetworkdownloads.py

@@ -27,6 +27,7 @@ import collections
 from PyQt5.QtCore import pyqtSlot, pyqtSignal, QTimer
 from PyQt5.QtNetwork import QNetworkRequest, QNetworkReply
 
+from qutebrowser.config import config
 from qutebrowser.utils import message, usertypes, log, urlutils, utils
 from qutebrowser.browser import downloads
 from qutebrowser.browser.webkit import http
@@ -366,7 +367,8 @@ class DownloadManager(downloads.AbstractDownloadManager):
     def __init__(self, win_id, parent=None):
         super().__init__(parent)
         self._networkmanager = networkmanager.NetworkManager(
-            win_id=win_id, tab_id=None, private=False, parent=self)
+            win_id=win_id, tab_id=None,
+            private=config.get('general', 'private-browsing'), parent=self)
 
     @pyqtSlot('QUrl')
     def get(self, url, *, user_agent=None, **kwargs):