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.
Reviewed-by: Sumit Bose <firstname.lastname@example.org>