Add SSH_CLIENT environment variable

Since the x.org rework, X has been installed into standard paths and not
its own random prefixes.  I think it's time we update the default paths
accordingly.

Try without identifying current user

Small change that warns the user if the current user cannot be identified rather than aborting.  This came in handy when I put dropbear on a dlink that did not have a true user environment.  Falling back on the "-l" option and user@ options works just fine as a client.  The only implication I found is that the -J option will fail ungracefully without a known own_user.

- Add release script
- Simplify print_version

-h should exit with success
Update manpages

if (connecting || ptys || x11) tos = LOWDELAY;
else if (tcp_forwards) tos = 0;
else tos = BULK;

TCP forwards could be either lowdelay or bulk, hence the default priority.

connection backlog, avoid check_close() for channels that haven't had IO