Various small fixes

This has various fixes for this;
Use $config['install_dir'] / $config['html_dir'] in all includes
Change remaining usage of function file_exists to is_file
Count down devices in device cache
Small html fixes in various pages
Use cache for some information in frontpage (ports, devices)
Remove remaining includes of print-device/interface-graph.php
Fixes sorting in devices page
Fix sql in locations page