core: make settings for unified cgroup hierarchy supersede the ones for legacy hierarchy (#4269)

There are overlapping control group resource settings for the unified and
legacy hierarchies.  To help transition, the settings are translated back and
forth.  When both versions of a given setting are present, the one matching the
cgroup hierarchy type in use is used.  Unfortunately, this is more confusing to
use and document than necessary because there is no clear static precedence.

Update the translation logic so that the settings for the unified hierarchy are
always preferred.  systemd.resource-control man page is updated to reflect the
change and reorganized so that the deprecated settings are at the end in its
own section.