[tac_plus] Debugging output cleanup

[Philp Prindeville]

This fixes the following things:

* dump characters as 2 hex digits, even if high nibble is zero;
* when displaying hash inputs, consistently show session_id in network 
byte order;
* dump hash (and prev_hash) on one line, rather than 1 byte per line;
* remove extraneous newline
* use open_memstream() to avoid strcat()'s, and other buffer-to-buffer 
copies;
* don't reinvent the printf-wheel. use vfprintf() instead;

Running it here locally.

-Philip


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