cgxget: Add a flag to ignore settings that can't be converted

cgxget: Add a flag to ignore settings that can't be converted

Add a flag, "-i", to cgxget to ignore settings that can't be
converted from cgroup v1 to cgroup v2 or vice versa.

Some cgroup v1 settings, e.g. cpuset.sched_relax_domain_level,
don't have a cgroup v2 equivalent and likely never will.  This
flag allows users to ignore errors from these unconvertable
settings.

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>