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).
@ktf 
I checked it with `rpmlint`, it was ok: mmhh :-(
Fixing it right now.
BTW, can you point me to the `cmsbuild` documentation: I found only sparse stuff in the Twiki.