log files used by neo storage are not text files, they are sqlite
databases, so we can not use copytruncate because it leaves a broken
database. For neo, we have to go back to sending a SIGUSR1 signal to
tell neo client to reopen log files. This depends on neo registering
a signal handler, which is done in [1]. It depends on ZServer, so this
approach will probably have to be adjusted when running on python3
because the current plan is that we don't have ZServer installed on
python3.

This depends on a recent erp5.git where runwsgi understands --pidfile
argument.

1: https://lab.nexedi.com/nexedi/neoppod/blob/fd87e153/neo/client/app.py#L58