Add ability to have post-download scripts in fog.  Update ipxe/undionly to latest pul.  Make ImageManagementPage not allow a file name postdownloadscripts or dev.

git-svn-id: https://svn.code.sf.net/p/freeghost/code/trunk@1945 71f96598-fa45-0410-b640-bcd6f8691b32