hterm 1.61 + sftp prep

• Bump min Chrome version to 41 since we don't test on anything older.
• Rebuilt ssh_client plugins to include some fd related changes.
• Update to hterm 1.61. Fixes missing last line, and adds clickable URLs.
• Include ssh man pages in html form for users to read.
• Fix --proxy-port passing for Google relays.
• Add Ctrl-Shift-P shortcut in crosh for opening preferences.
• Drop blocking of Ctrl-Shift-N shortcut in crosh.