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