Add support for a -d debug flag to enable debugging output.

Mostly intended for developer use.  At the moment there are no public
uses for this flag.

(cherry picked from commit cf59099b49474589075787fe79d12f71a4ca0e9e)
Signed-off-by: Bruce A. Mah <bmah@es.net>
Whoops.  Forgot to document the -d flag properly.  Do this.

(cherry picked from commit 7234074fe83d9571343c0e72cbe41411b66e3194)
Signed-off-by: Bruce A. Mah <bmah@es.net>
Output TCP snd_cwnd (on platforms supporting it) in normal output.

We were computing and printing this in JSON output mode anyway; this
change just exposes this quantity in a human-friendly manner (better
than the first attempt at this) when doing normal output.

Resolves #99 (Additional TCP_INFO items).

(cherry picked from commit d076653bd4a92a1587c8214a6e0603e25eda2307)
Signed-off-by: Bruce A. Mah <bmah@es.net>