BEE Backport regress/tests/fdcallsdir-many-clients-test

BEE Backport regress/tests/fdcallsdir-many-clients-test

This commit is the result of the squash of the following main commits:

Author: Eric Bollengier <eric@baculasystems.com>
Date:   Tue Jul 21 10:28:13 2020 +0200

    regress: Add copyright to regress scripts

Author: Alain Spineux <alain@baculasystems.com>
Date:   Wed Apr 10 10:43:27 2019 +0200

    regress: fix fdcallsdir-many-clients-test

Author: Eric Bollengier <eric@baculasystems.com>
Date:   Wed Mar 20 14:49:24 2019 +0100

    regress: Add fdcallsdir regress tests