Update L2TP to NetworkManager-l2tp 1.8.0 features

Summary:

• Update to NetworkManager-l2tp 1.8.0 features which include:
  • NetworkManager-l2tp 1.8.0 user certificate support.
  • Machine certificate support.
  • Rename Gateway ID to Remote ID.
  • Phase 1 & Phase 2 Lifetime TimeEdits.
  • Use IP Compression CheckBox.
  • Disable PFS CheckBox.
• Synchronize nm-l2tp-service.h with NetworkManager-l2tp 1.8.0's nm-service-defines.h.
• Base new user certificate UI on openvpn UI.
• Delete l2tpauth.ui and replace with programmatically generated l2tpauthwidget.
• Add user and machine private key / certificate password support to l2tpauthwidget.
• Detect if libreswan or strongswan is being used and change UI based on which one is detected, e.g. disable "Disable PFS" check box with strongswan, Phase 1 & Phase 2 Lifetime show default strongswan and libreswan values when unchecked.
• Disable IPSec settings button if libreswan or strongswan not detected.
• Remove visibility of PPP Authentication group box if user certificate authentication is selected.
• Support loading *SWAN Base64 encoded PSK, but for backwards compatibility use unencoded for saving settings.
• For backwards compatibility with older NetworkManager-l2tp use NM_L2TP_KEY_IPSEC_GATEWAY_ID instead of NM_L2TP_KEY_IPSEC_REMOTE_ID for saving connection settings.

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27764