Compatible with NetworkManager 1.20 and later.
Summary of changes since NetworkManager-l2tp 1.20.8 :
- Fix missing ppp user option.
- Add support for compiling against pppd-2.5.0, including pkgconfig support to pickup the pppd plugin directory.
- Add error output for missing TLS filenames.
- Remove bashisms in configure script.
- Spell checked code with CSpell.