netflow: annotate cache limit enforcement; closes #1170

While here, remove the RAM disk mention, we can cope with this now.

(cherry picked from commit f85f3566)
(cherry picked from commit 02f9143e)
(cherry picked from commit 47988b58)

src/opnsense/mvc/app/controllers/OPNsense/Diagnostics/forms/netflow_capture.xml

@@ -19,7 +19,7 @@
         <id>netflow.collect.enable</id>
         <label>Capture local</label>
         <type>checkbox</type>
-        <help>Collect NetFlow data on this firewall, this will automatically append localhost as NetFlow target. Please keep disabled when using a RAM disk (for size and consistency reasons).</help>
+        <help>Collect NetFlow data on this firewall for use with Insight. Note that the local cache only holds the latest 100 MB of data.</help>
     </field>
     <field>
         <id>netflow.capture.version</id>

src/www/reporting_settings.php

@@ -60,6 +60,9 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
 }
 
 $all_rrd_files = json_decode(configd_run("systemhealth list"), true);
+if (!is_array($all_rrd_files)) {
+    $all_rrd_files = array();
+}
 ksort($all_rrd_files);
 legacy_html_escape_form_data($pconfig);