Reimplement ad_page_contract_filter_proc_tmpfile using security::safe_tmpfile_p
Some of the features implemented by this filter have been ported into the api, namely the possibility to fetch the valid temp folders from the subsite TmpDir parameter and the possibility to relax the check and allow also files deeper in the tmpfolder hierachy. Notably, the hardcoded tmpfolders "/var/tmp" and "/tmp" have NOT been ported. One should configure these values via the many available options. security::safe_tmpfile_p is also more restrictive when a file exists, because it checks for ownership and read and write permissions on the file.