install.sh: show warning nonroot mode when systemd does not support user mode

On older distribution such as CentOS7, it does not support systemd user mode.
On such distribution nonroot mode does not work, show warning message and
skip running systemctl --user.

See scylladb/scylla#7071

install.sh

@@ -74,6 +74,11 @@ while [ $# -gt 0 ]; do
     esac
 done
 
+check_usermode_support() {
+    user=$(systemctl --help|grep -e '--user')
+    [ -n "$user" ]
+}
+
 if ! $packaging; then
     has_java=false
     if [ -x /usr/bin/java ]; then
@@ -151,7 +156,9 @@ if $nonroot; then
     sed -i -e "s#/var/lib/scylla#$rprefix#g" "$rsysconfdir"/scylla-jmx
     sed -i -e "s#/etc/scylla#$rprefix/etc/scylla#g" "$rsysconfdir"/scylla-jmx
     sed -i -e "s#/opt/scylladb/jmx#$rprefix/jmx#g" "$rsysconfdir"/scylla-jmx
-    systemctl --user daemon-reload
+    if check_usermode_support; then
+        systemctl --user daemon-reload
+    fi
     echo "Scylla-JMX non-root install completed."
 elif ! $packaging; then
     systemctl --system daemon-reload