Add a script to fix interfaces issue with Proxmox

Proxmox don't parse interfaces.d. See :
https://forum.proxmox.com/threads/anyway-to-support-interfaces-d.34739/

proxmox/build.network.file

@@ -0,0 +1,48 @@
+#!/bin/sh
+
+# Purpose : {{{
+# Proxmox only reads /etc/network/interfaces from a while
+#		See : https://forum.proxmox.com/threads/anyway-to-support-interfaces-d.34739/
+# So any bridge,… in the subdirectory /etc/network/interfaces.d won't be
+# available from webgui to manage KVM/LXC's network device.
+# }}}
+# This script will try to fix this waiting for a Proxmox's fix. {{{
+#		Check if /etc/network/interfaces.d contains file(s)
+#		Create a temp interfaces file
+#		Add some basic/header informations to the temp file
+#		Cat the content of interfaces.d files to the temp file
+#		Replace default interfaces file with the temp one
+
+# }}}
+
+interfaces_dir="/etc/network/interfaces.d"
+interfaces_temp="/etc/network/interfaces.temp.script"
+
+# Test if interfaces.d is empty
+if [ -z "$(ls -A ${interfaces_dir})" ]; then
+	exit 0
+fi
+
+# Create temp file
+rm -f -- "${interfaces_temp}"
+touch -- "${interfaces_temp}"
+
+# Header informations
+printf '%b' "# This file describes the network interfaces available on your system
+# and how to activate them. For more information, see interfaces(5).
+" >> "${interfaces_temp}"
+
+# Loopback informations if not present in interfaces.d
+grep -q -R "iface lo" "${interfaces_dir}" || printf '%b' "
+# The loopback network interface
+auto lo
+iface lo inet loopback
+" >> "${interfaces_temp}"
+
+# Cat the content of interfaces.d to temp file (POSIX way)
+find "${interfaces_dir}" -type f -exec cat {} >> "${interfaces_temp}" \;
+
+# Define it a new interfaces file
+mv -- "${interfaces_temp}" /etc/network/interfaces
+
+exit 0