lscpu: support offline CPUs

lscpu: support offline CPUs

 # echo 0 >/sys/devices/system/cpu/cpu3/online
 # echo 0 >/sys/devices/system/cpu/cpu2/online

 # grep processor /proc/cpuinfo
 processor       : 0
 processor       : 1

 # lscpu
 lscpu: error: cannot open
 /sys/devices/system/cpu/cpu2/cache/index0/shared_cpu_map: No such file or directory

This patch also add a new "On-line CPU(s):" line to the lscpu(1)
output.

Addresses: http://bugzilla.redhat.com/show_bug.cgi?id=623012
Signed-off-by: Karel Zak <kzak@redhat.com>