Debian postinst: check for present user/group before adding them

In git commit 7216aab0 the check for present user was removed
and therefore upgrading the Debian package on existing Jenkins
installations fails. Also the newly added code for adding a
configurable Jenkins group is missing a check for a possibly
already present Jenkins group.

Closes: JENKINS-22710

debian/debian/jenkins.postinst

@@ -25,15 +25,19 @@ case "$1" in
         : ${JENKINS_USER:=jenkins}
         : ${JENKINS_GROUP:=jenkins}
 
-        # Create jenkins user if it doesn't exist.
+        # Create jenkins group and user if they don't exist.
         # sometimes tools that users want Jenkins to run need a shell,
         # so use /bin/bash. See JENKINS-4830
-        addgroup --system --quiet $JENKINS_GROUP
-        adduser --system --quiet --home /var/lib/jenkins --no-create-home \
-            --ingroup $JENKINS_GROUP --disabled-password --shell /bin/bash \
-            --gecos 'Jenkins' \
-            $JENKINS_USER
-	
+        if ! getent group "$JENKINS_GROUP" > /dev/null; then
+            addgroup --system --quiet "$JENKINS_GROUP"
+        fi
+        if ! id "$JENKINS_USER" >/dev/null 2>&1 ; then
+            adduser --system --quiet --home /var/lib/jenkins --no-create-home \
+                --ingroup "$JENKINS_GROUP" --disabled-password --shell /bin/bash \
+                --gecos 'Jenkins' \
+                "$JENKINS_USER"
+        fi
+
         # If we have an old hudson install, rename it to jenkins
         if test -d /var/lib/hudson -a \! \( -e /var/lib/hudson/.for-jenkins \) ; then
             # leave a marker to indicate this came from Hudson.