idmap_autorid: factor out domain range adding code into a separate function

idmap_autorid: factor out domain range adding code into a separate function

This also adds a new mode to the new idmap_autorid_addrange() function
that allows to set a provided range if the range is available, instead
of the original only mode of automatically allocating a new range
by incrementing the HWM counter.

Pair-Programmed-With: Atul Kulkarni <atul.kulkarni@in.ibm.com>

Signed-off-by: Michael Adam <obnox@samba.org>
Signed-off-by: Atul Kulkarni <atul.kulkarni@in.ibm.com>
Reviewed-by: Volker Lendecke <vl@samba.org>