Fix tmp file mechanism

Force each process to maintain its own list of tmp files. If processes inherit this list, called processes, when calling sf_finish, delete files owned by calling processes.
Changes in the error system

The file containing the list of errors must be different for each process, and not inherited (potential conflict similar to tmp files).
Duplicate errors are now suppressed
SF_ERRLOG environment variable now allows to temporarily divert errors from the usual file.
New sf_error_import() allows to reinject those errors into the normal flow.
Add sf_sort_file()
Fix: warning message in sf_show_errors') when no error was raised during program execution (file does not exist).