Provide postinst to automatically execute ngcp-update-db-schema on 2.6 systems

This is supposed to help us getting the broken
/usr/share/ngcp-db-schema/db_scripts/diff/9868.up
script into existing 2.6 systems.

Needs excessive testing, not ready for release *yet*.

From: Michael Prokop <mprokop@sipwise.com>

debian/ngcp-db-schema.postinst

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+set -e
+
+case "$1" in
+    configure)
+        # apply bugfix on 2.6 ngcp systems with broken db rev script
+        # /usr/share/ngcp-db-schema/db_scripts/diff/9868.up
+        # ngcp 2.5 repos provides ngcp-db-schema := 0.1.3
+        # ngcp 2.6 repos provides ngcp-db-schema >= 2.6.3
+        if [ -n "$2" ] && dpkg --compare-versions $2 gt 0.1.3 && dpkg --compare-versions $2 le 2.6.3 ; then
+            echo "Executing ngcp-update-db-schema to address 2.6 db rev script bug."
+            ngcp-update-db-schema
+        fi
+    ;;
+
+    abort-upgrade|abort-remove|abort-deconfigure)
+    ;;
+
+    *)
+        echo "postinst of ngcp-db-schema called with unknown argument \`$1'" >&2
+        exit 1
+    ;;
+esac
+
+exit 0