smb: client: provide dns_resolve_{unc,name} helpers

Some places pass hostnames rather than UNC paths to resolve them to ip
addresses, so provide helpers to handle both cases and then stop
converting hostnames to UNC paths by inserting path delimiters into
them.  Also kill @expiry parameter as it's not used anywhere.

Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>