system: move authgui.inc to appropriate location

Auth and gui were already split but stored side by side but it makes
no sense for authgui to be used anywhere other than the legacy GUI.

plist

@@ -9,7 +9,6 @@
 /usr/local/etc/inc/IPv6.inc
 /usr/local/etc/inc/XMLRPC_Client.inc
 /usr/local/etc/inc/auth.inc
-/usr/local/etc/inc/authgui.inc
 /usr/local/etc/inc/certs.inc
 /usr/local/etc/inc/config.inc
 /usr/local/etc/inc/console.inc
@@ -2063,6 +2062,7 @@
 /usr/local/share/man/man8/opnsense-version.8.gz
 /usr/local/wizard/openvpn.xml
 /usr/local/wizard/system.xml
+/usr/local/www/authgui.inc
 /usr/local/www/crash_reporter.php
 /usr/local/www/csrf.inc
 /usr/local/www/diag_authentication.php