Make explicit the access to different storage areas in received file paths

[grololo06]

The current convention is that:
e.g. /tmp/tmpj6jglurb.3/ is a temporary directory containing a single file, short-lived with unpredictable name
e.g. /tmp/ecotaxa_user.000/ is a semi-permanent (until next reboot!) directory corresponding to calls to my_files entry points
Any other directory means "sub-directory of the common one"
Formalizing the different cases with a prefix would be clearer, e.g. temp: user: common: