[server] use sqls_create_multicol_unique_index to index entities.extid SQL Server's UNIQUE is not standards compliant, it doesn't accept two NULL values. Related to #7173702.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/python2*/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl