Support for /etc/apt/apt.conf.d/20auto-upgrades

Checks /etc/apt/apt.conf.d/20auto-upgrades first for proper configuration.
If it doesn't exist, checks /etc/apt/apt.conf.d/10periodic instead for compatibility with previous unattended updates configuration process.

Include instructions for installation on remote hosts.

The title says it all.

Display upgrade period

The plugin will now not only check that unattended_upgrades is configured to run correctly, but that it is configured to run. It will also display in the nagios admin how many days it waits in between runs.

Initial Release

Works well with full checks of package installation and configuration.