display the max file size to the user

this should ideally take the minimum of $g_max_file_size and the php variables upload_max_filesize and post_max_size but the last two seem to be returned as strings (eg '2M') on my system

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1284 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9