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