idmap: allow first RID to be set
Currently libss_idmap implicitly assumes that the RID 0 is always mapped
to the first ID of the given range. This is not the case anymore when
multiple ranges are used e.g. for trusted domains in FreeIPA.
A new call sss_idmap_add_domain_ex() was added which can take the first
RID as an argument. This new call will get more options with other
patches hence I didn't change the library version with this patch.
Fixes https://fedorahosted.org/sssd/ticket/1938