New function ("svc_uses_rpcbind") will be used to detect, that new service will
send portmapper register calls. For such services we will create rpcbind
clients and remove all stale portmap registrations.
Also, svc_rpcb_cleanup() will be set as sv_shutdown callback for such services
in case of this field wasn't initialized earlier. This will allow to destroy
rpcbind clients when no other users of them left.

Note: Currently, any creating service will be detected as portmap user.
Probably, this is wrong. But now it depends on program versions "vs_hidden"
flag.
Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>