core: log cgroup legacy and unified hierarchy setting translations

core: log cgroup legacy and unified hierarchy setting translations

To accommodate changes in kernel interface, cgroup unified hierarchy support
added several configuration items which overlap with the existing resource
control settings and there is simple config translation between the overlapping
settings to ease the transition.  As why certain cgroup knobs are being
configured can become confusing, this patch adds a master warning message which
is printed once when such translation is first used and logs each translation
with a debug message.

v2:

- Switched to log_unit*().