Provide a fallback ROOT_PATH

If unable to detect the root path, provide a fallback ROOT_PATH setting to
'./'. This is likely to happen if run from the commandline (like for crons)
or if DOCUMENT_ROOT and the folder of main.inc.php seem to have nothing in
common

Fixes #704
Merge pull request #705 from greezybacon/issue/root-path-cli

Provide a fallback ROOT_PATH

Reviewed-By: Peter Rotich <peter@osticket.com>