libknc.c: add knc_set_net_fd() and use it internally.
libknc.3: more documentation.
libknc.c: add preliminary support for sending out of band commands.

We add preliminary support for sending commands over the wire so that
each side can negotiate various options.  For now, we support only a
single command which closes the connexion.
libknc: add knc_is_established() to test if the connexion is passing data

or if it is still in the authentication phases.
libknc.c: remove a comment that no longer applies.
libknc.h: remove unused function prototype.
libknc: major API rototill.

We are starting to make a major rototill to the libknc API to bring it
into a state where we can commit to it.  This commit introduces new
constructors, removes old constructors, allows one to set and query
various properties of the knc_ctx, renames struct knc_ctx * into knc_ctx,
and much else.  It is not quite complete insofar as the API is still
not exactly that to which we would prefer to commit, however, it is
much closer and the [modified] unit test still works.