Further separation of mnemon main program from mnemon library.

Further separation of mnemon main program from mnemon library.

As I'm starting to generate a real interface here, I continue to find
functionality that would violate that interface. Fix this by moving things
up into the main program as necessary to keep the interface fairly clean.

For example, all session-based notions, (such as to_introduce,
to_master, etc.), are now part of a new progress_t within main.c
instead of being part of mnemon_t in the library, (since one program
might have a vey different notion of what a session is than another).