lscpu: add --offline option

lscpu: add --offline option

Implement "--offline" option which only prints offline cpus. As a side effect
we can get rid of the internal "allcpus" flag, since if we want to print
informations for online and offline cpus we simply set both flags.

When reading sysfs attributes of cpus this is now done for all cpus, since
e.g. the topology informations of the online cpus may influence the
topology informations of the offline cpus. This mainly because online cpus
may contain masks which include offline cpus while offline cpus have a
missing topology directory.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>