idmap: Initialize all idmap domains at startup

idmap: Initialize all idmap domains at startup

So far we have initialized idmap domains on demand indexed by name.
For sid2xid this works okay, because we could do lookupsids before
and thus get the name. For xid2sid this is more problematic. We
have to rely on enumtrustdoms to work completely, and we have to
look at the list of winbind domains in the parent to get the domain
name. Relying on domain->have_idmap_config is not particularly nice.

This patch re-works initialization of idmap domains by scanning all
parametric parameters, scanning for :backend configuration settings.
This way we get a complete list of :range definitions. This means
we can rely on the idmap domain array to be complete. This in turn
means we can live without the domain name to find a domain, we can
do a range search by uid or gid.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Bug: https://bugzilla.samba.org/show_bug.cgi?id=11464