examples/nfs-ls: add discovery mode

this adds the ability for nfs-ls to discover

a) local NFS servers

examples/nfs-ls -D nfs://

b) NFS shares on a server

examples/nfs-ls -D nfs://10.0.0.1

c) all shares on local NFS servers

examples/nfs-ls -D -R nfs://

Enjoy!

Signed-off-by: Peter Lieven <pl@kamp.de>