Add a new CommandResult code called "FINISH" which, unlike "CLOSE",
will attempt to flush the output buffer.  This is a one-shot attempt;
it will do one write, and not try again.

Remove the const_cast from HttpdClient.cxx, and avoid one allocation
in ClientRead.cxx.

Don't use a global loop over the whole client list.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The soure file client.c has nearly 1000 lines, time for splitting it
into smaller pieces to improve readability.