debian: avoid lintian maintainer-script-empty

We are going to fill in these files with useful things.

debian/freeswitch.preinst

@@ -3,6 +3,9 @@ set -e
 
 case "$1" in
   install|upgrade)
+    if [ -d /opt/freeswitch ]; then
+      echo "It looks like you have FreeSWITCH installed manually." >&2
+    fi
     ;;
 
   abort-upgrade)

debian/freeswitch.prerm

@@ -3,6 +3,7 @@ set -e
 
 case "$1" in
   remove|upgrade|deconfigure)
+    echo "We're about to remove FreeSWITCH..." >&2
     ;;
 
   failed-upgrade)