Use vt 1 when systemd support is available

systemd graphical mode uses vt 1 for display managers.
Add missing USE_PAM definition to Authenticator.cpp
Always go via PAM for autologin

This passes our session through the systemd PAM module that sets up
logind correctly, fixing a bug where polkit actions would not work
correctly in an SDDM session
Provide seat name and virtual terminal number to PAM environment variables
remove pointless pam checks in autologin
Rewrote the config to a more rigid state

Resolves sddm/sddm#39

More configuration instances now available. Every instance is a separate class.

Sections are supported.

Serialization handled in a more sensitive way - does not remove comments.

Type handling is unified, using the QTextStream shift operators.

There are also some basic tests, to be further improved...

Probably the most overengineered configuration parser you've ever witnessed.