Fix 0011534: Incorrect path/short_path detection

Rather than attempting to try and decipher the path and short_path by
comparing the file's location on the filesytem with the document_root,
MantisBT should be looking at the PHP_SELF server var provided by PHP.
This variable is based on the current URL used to access the file, and
is not confused by symlinks or the document_root pointing directly to
the installation. It is also much simpler than the old method.

Tested on Linux and Windows with the following setups:

• installed as subdomain, eg http://mantis.host/foo.php
• installed in subdirectory, eg http://host/mantisbt/foo.php
• accessed by symlink (Linux), eg http://host/mantis/foo.php
  where /var/www/mantis -> /usr/share/mantisbt