d/maintscripts: use postinst arguments to determine first install

Following https://www.debian.org/doc/debian-policy/ap-flowcharts.html
postinst gets called as 'postinst configure' w/o second argument on
first installation, use that information instead of creating a
flag-file in preinst.

Technically this changes the semantics and the first-install parts
will not be run, in case ifupdown2 had been installed on the system,
but was subsequently removed, but not purged ('rc' in dpkg-output).
Since the functionality was added quite a while ago (released with PVE
6.2 to support ovs-setups with ifupdown2 - the potential for
regression should be limited. Originally introduced in:
849ae55de6

Fixes: 8d5303c350
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

debian/ifupdown2.postinst

@@ -111,11 +111,10 @@ case "$1" in
         process_udev
         chmod +x /usr/share/ifupdown2/__main__.py
         postinst_remove_diverts
-        if [ -f "/tmp/.ifupdown2-first-install" ] && [ ! -e /proxmox_install_mode ]; then
+        if [ -z "$2" ] && [ ! -e /proxmox_install_mode ]; then
             proxmox_compatibility
             echo "Reloading network config on first install"
             ifreload -a || report_warn "Reloading failed"
-            rm  /tmp/.ifupdown2-first-install
         fi
     ;;
 

debian/ifupdown2.preinst

@@ -1,20 +0,0 @@
-#!/bin/sh
-set -e
-
-case "$1" in
-    install)
-        touch /tmp/.ifupdown2-first-install
-        ;;
-
-    upgrade|abort-upgrade)
-        ;;
-
-    *)
-        echo "postinst called with unknown argument \`$1'" >&2
-        exit 0
-        ;;
-esac
-
-#DEBHELPER#
-
-exit 0