More warns cleanups and misc cleanups:

1. Make fetch_mode a pointer 'cos we can just use the optarg.
2. Constness.
3. Add/complete prototypes.
4. Change an int to a socklen_t.
5. Don't use C++ style comments.
6. Check return values from read and fork a little more carefully.
7. Avoid closing an uninitialised int.
8. Get rid of unneeded extern declarations.