Bug 25481: Pass --user to start-stop-daemon when a pidfile is used

Since D10, the behaviour of start-stop-daemon changed, see from its
manual:
"""
Warning:  using this match option with a world-writable pidfile or using it alone with a daemon that writes the pidfile as an unprivileged (non-root) user will be refused with an error (since
version 1.19.3) as this is a security risk, because either any user can write to it, or if the daemon gets compromised, the contents of the pidfile cannot be trusted, and  then  a  privileged
runner (such as an init script executed as root) would end up acting on any system process.  Using /dev/null is exempt from these checks.
"""

Test plan:
koha-plack --restart kohadev
should success with this patch.
Without this patch you get:
start-stop-daemon: matching only on non-root pidfile /var/run/koha/kohadev/plack.pid is insecure

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

debian/scripts/koha-functions.sh

@@ -269,6 +269,7 @@ is_plack_running()
     local instancename=$1
 
     if start-stop-daemon --pidfile "/var/run/koha/${instancename}/plack.pid" \
+            --user="$instancename-koha" \
             --status ; then
         return 0
     else
@@ -292,6 +293,7 @@ is_z3950_running()
     local instancename=$1
 
     if start-stop-daemon --pidfile "/var/run/koha/${instancename}/z3950-responder.pid" \
+            --user="$instancename-koha" \
             --status ; then
         return 0
     else

debian/scripts/koha-plack

@@ -144,7 +144,7 @@ stop_plack()
 
         log_daemon_msg "Stopping Plack daemon for ${instancename}"
 
-        if start-stop-daemon --pidfile ${PIDFILE} --stop --retry=TERM/30/KILL/5; then
+        if start-stop-daemon --pidfile ${PIDFILE} --user="${instancename}-koha" --stop --retry=TERM/30/KILL/5; then
             log_end_msg 0
         else
             log_end_msg 1