logind: load SELinux labelling system

elogind-logind uses mkdir_label and label_fix functions without calling
first mac_selinux_init.  This makes /run/user/$UID/ directories not
labelled correctly on an Arch Linux system using SELinux.

Fix this by calling mac_selinux_init("/run") early in elogind-logind.
This makes files created in /etc/udev/rules.d and /var/lib/elogind to be
labelled through transitions in the SELinux policy instead of using
setfscreatecon (with mac_selinux_create_file_prepare).