[MINOR] term_trace: add better instrumentations to trace the code

[MINOR] term_trace: add better instrumentations to trace the code

A new member has been added to the struct session. It keeps a trace
of what block of code performs a close or a shutdown on a socket, and
in what sequence. This is extremely convenient for post-mortem analysis
where flag combinations and states seem impossible. A new ABORT_NOW()
macro has also been added to make the code immediately segfault where
called.