idmap: Factor out lp_scan_idmap_domains()

This simplifies idmap_found_domain_backend() by moving the regex magic
somewhere else. Also, this routine will be useful soon somewhere else, thus
make it non-static to idmap.c.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>