apc discovery: Get a count of banks before pulling info

* On my device (AP7901 / AOS v3.9.0 / App v3.7.4) discovery would keep
  clobbering the bank and current sensor.  Looking in to it the oid
  should be used for current and not bank.  To fix this now it pulls
  a count of banks using the `rPDULoadDevNumBanks.0` oid. Per MIB that
  will return 0 if no banks are found.  In which case skip the bank
  detection.

* Also update PowerNet-MIB to v4.1.7 released 2015-08-05

includes/discovery/sensors/current/apc.inc.php

@@ -58,7 +58,10 @@ if ($device['os'] == 'apc') {
     unset($oids);
 
     // v2 firmware- first bank is total, v3 firmware, 3rd bank is total
-    $oids = snmp_walk($device, 'rPDULoadStatusIndex', '-OsqnU', 'PowerNet-MIB');
+    $bank_count = snmp_get($device, 'rPDULoadDevNumBanks.0', '-Oqv', 'PowerNet-MIB');
+    if ($bank_count > 0) {
+        $oids = snmp_walk($device, 'rPDULoadStatusIndex', '-OsqnU', 'PowerNet-MIB');
+    }
     // should work with firmware v2 and v3
     if ($oids) {
         echo 'APC PowerNet-MIB Banks ';