Check if running kernel differs from installed kernel

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Improved apt-show-versions parsing logic

Signed-off-by: Samuli Seppänen <samuli.seppanen@gmail.com>
Updated LICENSE

Signed-off-by: Samuli Seppänen <samuli.seppanen@gmail.com>
Updated running/installed kernel checks

There were two problems with the previous implementation

- When using a backported kernel it's quite difficult to figure out the
  correct kernel metapackage name from uname output. Made the script bail out
  in these cases instead of crashing with UnboundLocalError.
- Isolating the kernel version from apt-show-versions output proved too fragile.
  Moved to comparing the running kernel version to entire apt-show-versions
  output.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Migrated to using update-notifier

The use of uname and apt-show-versions for detecting if a reboot was needed
proved to be too fragile. Started using the standard Debian/Ubuntu tools in
update-notifier-common package to determine how many pending updates and
security updates there are, as well as whether the computer needs to be
rebooted. The only minor issue with this approach is that reboot check will not
work until at least one kernel has been installed after the
update-notifier-common install.

Signed-off-by: Samuli Seppänen <samuli.seppanen@gmail.com>
Signed-off-by: Samuli Seppänen <samuli@openvpn.net>