[FIX] picking of username for session data directory in case of setuid

Try using geteuid() before getpass.getuser() so a system where
environment user and effective user (e.g. sudo, start-stop-daemon)
creates session-storage directory with a username matching the access
rights.

lp bug: https://launchpad.net/bugs/1172306 fixed

bzr revid: xmo@openerp.com-20130610092107-o4ym83e4ycz450un