Moved all threading code into threads.c

Moved all threading code into threads.c
Slight alteration to worker thread startup code.
Altered the order of the shutdown so that we shutdown the modules and
the ports before closing the database. I'm curious about the remaining
locks in the data base after shutdown.

Still to come.
Wrappers for the pthread calls so that they can return to sysdep.c (the
portability file).
Further speed improvements.
Module to get thread data for user display.