util: add find_subdomain_by_sid()
This function takes domain SID (doesn't have the last component)
or object SID (have all components) and returns subdomain.
The subdomain is found by comparing domain->domainid with the SID.
E.g.
domain SID: S-1-5-21-3940105347-3434501867-2690409756
object SID: S-1-5-21-3940105347-3434501867-2690409756-513
Resolves:
https://fedorahosted.org/sssd/ticket/2034