lib/sysfs: add subsystem and hotplug detection

Signed-off-by: Karel Zak <kzak@redhat.com>
eject: use sysfs API to detect hotplug

Signed-off-by: Karel Zak <kzak@redhat.com>
lsblk: add HOTPLUG column

Signed-off-by: Karel Zak <kzak@redhat.com>
docs: update TODO file

Signed-off-by: Karel Zak <kzak@redhat.com>
lsblk: add SUBSYSTEMS column

 $ lsblk -oNAME,SIZE,TYPE,SUBSYSTEMS /dev/sr0
 NAME  SIZE TYPE SUBSYSTEMS
 sr0   7.8M rom  block:scsi:usb:pci

Signed-off-by: Karel Zak <kzak@redhat.com>