MDL-52766 mnet: Support MNet peer URLs longer than 64 characters

The commonName in SSL certificate is limited to 64 characters as per
RFC 5280 (https://www.ietf.org/rfc/rfc5280.txt). We respect that limit
when generating the CN attribute from the site's $CFG->wwwroot. But then
we did not respect it when comparing the common name with the peer's
URL so the certificate was not considered valid.

mnet/peer.php

@@ -174,7 +174,7 @@ class mnet_peer {
             $a['host'] = $this->wwwroot;
             $this->error[] = array('code' => 5, 'text' => get_string("nonmatchingcert", 'mnet', $a));
             return false;
-        } elseif ($credentials['subject']['CN'] != $this->wwwroot) {
+        } else if ($credentials['subject']['CN'] !== substr($this->wwwroot, 0, 64)) {
             $a['subject'] = $credentials['subject']['CN'];
             $a['host'] = $this->wwwroot;
             $this->error[] = array('code' => 4, 'text' => get_string("nonmatchingcert", 'mnet', $a));