389/389_useradd.sh: Add copyright and GPL license header

Original script is copyright Red Hat, but was disitributed without
a license[0], so I've just included a note that portions of the sc-
ript are under their copyright.

[0] https://www.redhat.com/resourcelibrary/whitepapers/netgroupwhitepaper
389: Add copy of GPLv3 license