[tac_plus] Fixing single connect mode

[Philip Prindeville]

If anyone is interested, there were some bugs that impeded single connection mode from working.

This commit fixes that:

https://github.com/pprindeville/tac_plus/commit/b71502fac3ee593468c87bd4253eac423fc6ed70

The main problems were that we were checking for the seq_no for being 1 during authentication or authorization requests; and we were resetting the session sequence number each time through start_session()’s loop.  The latter should only have happened at the top of the loop.

The only verification needed is that the received header’s seq_no needs to match that of the session.

The changes are trivial.

-Philip