core: make SYSTEMD_CGROUP_CONTROLLER a special string

SYSTEMD_CGROUP_CONTROLLER is currently defined as "name=elogind" which cgroup
utility functions interpret as a named cgroup hierarchy with the specified
named.  With the planned cgroup hybrid mode changes, SYSTEMD_CGROUP_CONTROLLER
would map to different hierarchy names.

This patch makes SYSTEMD_CGROUP_CONTROLLER a special string "_elogind" which is
substituted to "name=elogind" by the cgroup utility functions.  This allows the
callers to address the elogind hierarchy without actually specifying the
hierarchy name allowing the cgroup utility functions to map it to whatever is
appropriate.

Note that SYSTEMD_CGROUP_CONTROLLER was already special on full unified cgroup
hierarchy even before this patch.