library: lift 1024 byte restriction on control groups

library: lift 1024 byte restriction on control groups

The control group hierarchies for any particular task
could conceivably grow quite large.  However, the
library might impose an arbitrary limit of 1024 bytes
via fill_cgroup_cvt.

Two utility buffers of 128 KiB each were already
available for command line use.  This commit simply
trades the smaller 1024 byte stack based buffers for
those much larger existing ones.  Thus, truncation
can be avoided with no additional run-time costs.

Signed-off-by: Jim Warner <james.warner@comcast.net>