Use standard installation paths

CMake already has support for standard installation paths
including GNUInstallDirs.  Remove our definitions and use theirs.

While we are here, use a more conventional path for data files.
Merge pull request #189 from plfiorini/cmakeinstalldirs

Use standard installation paths
Fix configuration file path

Use absolute path for the configuration file otherwise sddm won't
find it.
Set default paths for SYSCONFDIR and LOCALSTATEDIR

GNUInstallDirs defines relative paths, when asked for absolute paths
like we do here it just prepends CMAKE_INSTALL_PREFIX resulting in paths
like /usr/etc and /usr/var for SYSCONFDIR and LOCALSTATEDIR
respectively.

However this is far from what we want, plus I find it unconvinient
to ask packagers and developers to pass arguments for those path
every time when we know the vast majority of people want /etc and
/var.
Use GNU installation directories for more files

There were a few places where we were still using semi-hardcoded
paths.
Fix man pages installation

MAN1_OUTPUT and MAN5_OUTPUT are meant to be lists so they shouldn't be
quoted.
Fix sddm daemon path in systemd unit
Merge pull request #190 from plfiorini/fixcmake

Fix CMake paths problems
Check chown return value

Print an error and quit when we cannot chown the socket.
Merge pull request #193 from ilpianista/master

Check chown return value
Add QAuth library and helper

Currently SDDM provides only a simple username/password authentication.
More exciting credential requests such as password change are not supported.

PAM also requires double forking to work better and that's not currently
implemented.

Importing QAuth here opens a lot of possibilities.

More changes to SDDM are required to handle more complicated credential
requests, for now we just want to start using QAuth to gain the same
features as current SDDM. We will improve as we move forward.

More details: https://github.com/sddm/sddm/wiki/Authentication
Use QAuth for authentication

Now that SDDM includes QAuth we can take advantage of its improved
authentication support.

This is based on PR #155 by Martin Bříza <mbriza@redhat.com>
Improve greeter stderr and stdout logging from daemon

Unquote output and avoid extra spaces.
Use sddm PAM service in QAuth's PAM backend

Use our naming for PAM services.
Keep in mind that we don't have the sddm-check PAM service yet.
Use message handler for sddm-helper

Log just like sddm-daemon and sddm-greeter.
Also remove redundant parts from the log messages.
Merge pull request #191 from plfiorini/qauth

Improved authentication mechanism with QAuth
Don't pad vt number with zeros

It's not required, plus other display managers don't pad with zeros.
Make the greeter go through logind

Use QAuth for the greeter session so that it will be registered
as a logind session with class "greeter".

With this, logind can pass drm devices to the greeter,
resolving all permission denied errors.

This work will also be needed by the Wayland port.

From now on we don't support running the greeter as root,
however the old code is preserved for test mode.
QAuth: Don't override XAUTHORITY every time

Setting XAUTHORITY makes sense only if DISPLAY is not and we don't want
to override any previous value to preserve what greeter set.
QAuth: Use sddm-greeter service for the sddm user

Use the sddm-greeter PAM service if we are authenticating the sddm
user.