Execute pihole through wrapper

All different exec() calls basically just calls sudo pihole,
with various different syntaxes. Using a wrapper function
allows sanitization of arguments for better safety.

Signed-off-by: Samu Voutilainen <smar@smar.fi>

scripts/pi-hole/php/groups.php

@@ -746,5 +746,6 @@ if ($_POST['action'] == 'get_groups') {
 }
 // Reload lists in pihole-FTL after having added something
 if ($reload) {
-    echo shell_exec('sudo pihole restartdns reload-lists');
+    $output = pihole_execute('restartdns reload-lists');
+    echo implode("\n", $output);
 }