Improve protocol robustness and invoke user callbacks.

Improve protocol robustness and invoke user callbacks.

All aborts are removed, as well as all asserts which aren't
programming errors.

When an invalid packet is received, the connection is closed, as per
draft-ietf-radext-tcp-transport-08 (2.6.4).

Use new rs_debug() macro rather than fprintf() for debug printouts.

Coding style overhaul.