Show private flag in --show-files output - #2367

aria2c -S or --show-files already parses the torrent's private flag internally (TorrentAttribute::privateTorrent), but it does not display it in the output.

This patch adds a Private: yes/no line to the BitTorrent file information so that users can immediately verify whether a torrent is marked as private.

Example output:

$ aria2c --show-files private.torrent
>>> Printing the contents of file 'private.torrent'...
*** BitTorrent File Information ***
Mode: single
Private: yes
Announce:
 <removed>
Info Hash: <removed>
Piece Length: 16KiB
The Number of Pieces: 1
Total Length: 57B (57)
Name: testfile.bin
Magnet URI: magnet:?<removed>
Files:

Fixes #2297.