Add nagios plugin documentation and example files.

README.nagios.md

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+needrestart - nagios plugin mode
+================================
+
+Needrestart can be used as a nagios plugin:
+
+```console
+# needrestart -p
+CRIT - Kernel: 3.16.0-4-amd64, Services: none, Containers: 1 (!), Sessions: none|Kernel=0;0;;0;2 Services=0;;0;0 Containers=1;;0;0 Sessions=0;0;;0
+```
+
+Since needrestart requires root privileges to scan processes of other
+users you should use sudo. Needrestart ships some example files to run
+needrestart as nagios plugin using sudo:
+
+- `ex/nagios/check_needrestart` - calls sudo to invoke needrestart
+- `ex/nagios/needrestart` - sudo(8) config allowing nagios to run needrestart as root
+- `ex/nagios/check_needrestart.conf` - nagios(8) integration

ex/nagios/check_needrestart

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+#!/bin/sh
+
+# needrestart - Restart daemons after library updates.
+#
+# Authors:
+#   Thomas Liske <thomas@fiasko-nw.net>
+#
+# Copyright Holder:
+#   2013 - 2015 (C) Thomas Liske [http://fiasko-nw.net/~thomas/]
+#
+# License:
+#   This program is free software; you can redistribute it and/or modify
+#   it under the terms of the GNU General Public License as published by
+#   the Free Software Foundation; either version 2 of the License, or
+#   (at your option) any later version.
+#
+
+exec sudo -n -- /usr/sbin/needrestart -p

ex/nagios/needrestart-nagios

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+# needrestart nagios plugin sudoers.d config file
+# -----------------------------------------------
+#
+# Please consider a look at /etc/sudoers.d/README howto enable this file.
+#
+
+# Allow nagios to execute the needrestart command
+nagios		ALL=NOPASSWD: /usr/sbin/needrestart -p

ex/nagios/plugin.conf

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+## remember: check_needrestart is a local check only
+define command {
+  command_name	        check_needrestart
+  command_line	        /usr/lib/nagios/plugins/check_needrestart
+}
+
+## example service
+#define service {
+#  host_name             localhost
+#  service_description   NEEDRESTART
+#  check_command         check_needrestart
+#  use                   generic-service
+#}