create expected user if it doesn't exist

in recent raspbian images pi user doesn't exist by default

src/main/deb/postinst

@@ -1,6 +1,20 @@
 chmod 755 ${config.installDir}/lib/download.sh
 ${config.installDir}/lib/download.sh ${config.installDir}/lib/ /usr/share/java/r2cloud/
 
+if id "${config.user}" &>/dev/null; then
+    echo "User '${config.user}' already exists."
+else
+    useradd -m ${config.user}
+    echo "User '${config.user}' created."
+
+    if ! getent group ${config.group} > /dev/null; then
+        groupadd ${config.group}
+        echo "Group '${config.group}' created."
+    fi
+
+    usermod -aG dialout,plugdev ${config.user}
+fi
+
 chown -R ${config.user}:${config.group} ${config.installDir} /usr/share/java/r2cloud/*.jar
 chmod 640 ${config.installDir}/lib/*.jar /usr/share/java/r2cloud/*.jar