ifupdown2: fixup maintainer scripts

Ticket: CM-5764
Reviewed By: TBA
Testing Done: of course none ;) Will update when I have some time to test

fupdown2's postinst and postrm scripts do not conform to Debian's policy
for maintainer scripts [1]. Specific non-conformance follows:

* fixed maintainer scripts so they only use /bin/sh as other shells are
  non-essential packages and may not exist on the system
* fixed maintainer scripts so only actions specified by $1 are performed
  per run of the script
* fixed maintainer scripts to only support specific values for $1, and
  will otherwise exit with error
* included at the bottom of the postrm script the "#DEBHELPER#" tag as
  during package building debianhelper will inject extra code at this
  location

[1] https://www.debian.org/doc/debian-policy/ch-maintainerscripts.html
(cherry picked from commit 2289fe4e90a0134fcb07fe7e2e838de755d73e68)

debian/python-ifupdown2.postrm

@@ -1,20 +1,57 @@
-#!/bin/bash
+#!/bin/sh
+# postrm script for ifupdown2
+#
+# see: dh_installdeb(1)
 
 set -e
 
-rm -f /sbin/ifup /sbin/ifdown /sbin/ifquery
+# summary of how this script can be called:
+#        * <postrm> `remove'
+#        * <postrm> `purge'
+#        * <old-postrm> `upgrade' <new-version>
+#        * <new-postrm> `failed-upgrade' <old-version>
+#        * <new-postrm> `abort-install'
+#        * <new-postrm> `abort-install' <old-version>
+#        * <new-postrm> `abort-upgrade' <old-version>
+#        * <disappearer's-postrm> `disappear' <overwriter>
+#          <overwriter-version>
+# for details, see http://www.debian.org/doc/debian-policy/ or
+# the debian-policy package
+
+postrm_remove()
+{
+	rm -f /sbin/ifup /sbin/ifdown /sbin/ifquery
+	update-rc.d networking remove >/dev/null
+}
+
+# Note: We don't remove /etc/network/interfaces
+postrm_purge()
+{
+	rm -f /var/tmp/network/ifstatenew
+	if [ -L /etc/network/run ] ; then
+		rm -f /etc/network/run
+	elif [ -d /etc/network/run ] ; then
+		rmdir --ignore-fail-on-non-empty /etc/network/run
+	fi
+}
 
 case "$1" in
-  purge)
-    # Note: We don't remove /etc/network/interfaces
-    rm -f /var/tmp/network/ifstatenew 
+	purge)
+		postrm_purge
+	;;
 
-    if [ -L /etc/network/run ] ; then
-      rm -f /etc/network/run
-    elif [ -d /etc/network/run ] ; then
-      rmdir --ignore-fail-on-non-empty /etc/network/run
-    fi
-    ;;
+	remove)
+		postrm_remove
+	;;
+
+
+	upgrade|disappear|failed-upgrade|abort-install|abort-upgrade)
+	;;
+
+	*)
+		echo "postrm called with unknown argument \`$1'" >&2
+		exit 1
+	;;
 esac
 
 udevlink=$(readlink /etc/udev/rules.d/80-networking.rules 2>/dev/null || true)
@@ -22,6 +59,9 @@ udevlink=$(readlink /etc/udev/rules.d/80-networking.rules 2>/dev/null || true)
 udevlink=$(readlink /etc/udev/rules.d/60-bridge-network-interface.rules 2>/dev/null || true)
 [ -n "$udevlink" -a "$udevlink" == "/dev/null" ] && rm -f /etc/udev/rules.d/60-bridge-network-interface.rules
 
-if [ "$1" = "purge" ] ; then
-    update-rc.d networking remove >/dev/null
-fi
+# dh_installdeb will replace this with shell code automatically
+# generated by other debhelper scripts.
+
+#DEBHELPER#
+
+exit 0