Add usage message

If you are interested in a basic usage message, here you go. If you are interested in a man page I can do that too. 

note: I am still trying to wrap myself around this code. What I had kind of expected was more AHCP-like,
where you would specify something like

hnetd --server --ip6prefix 2001:a:b:c::/48 --ip4prefix 10.1.0.0/20 --ntpserver fd::1/128 eth0 wlan0

and a client would look like:

hnetd --client -6 --ip6want 64 eth0 eth1 eth2 eth3 # which would ask for ipv6 only, and combine the requests into a /62 request if possible.

Not clear to me is a relay mode either (where a router would redistribute hnetd info but not allocate any
addresses itself.

Still reading...