RESOLV: Add a resolv_hostport_list request
    
    Adds a request that resolves a list of (host,port) tuples and returns a
    list of structures that contain the resolv_hostent structure as other
    resolver requests do, but also a pointer to the original request tuple.
    
    This is done because the request skips any unresolvable inputs, so it
    might be handy to know which input an output maps to.
    
    It is expected that the request will be used in the future also for cases
    where we want to e.g. try the connectivity to a serve using a mechanism
    such as an LDAP ping.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3291
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>