[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.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances