fasthadd file permissions

The new fastHadd external deals correctly with file permissions, by setting them to `0664`: this allows for testing/debugging purposes (files are accessible by all users), and for correctly handling files in `DAQ2` (while in the same step, files are handled by the same user, when moved in the data flow chain, files should be handled by users in the same group).
all we have is in:

https://github.com/cms-sw/pkgtools/tree/V00-22-XX/docs

and it's admittedly sparse. Will try to improve it as I find time.