netcmd: Use next_free_rid() function to calculate a SID for restoring a backup

netcmd: Use next_free_rid() function to calculate a SID for restoring a backup

This means we won't get errors if the DC doesn't have a rIDNextRID
attribute, but we will still error if there is no RID Set or if all its
pools are exhausted.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14669

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
(cherry picked from commit 59d293b60608172ae61551c642d13d3b215924e4)