- Making various functions return int rather than size_t.

- Making various functions return int rather than size_t.
  (Undoing a recent change).  Since size_t is unsigned on
  Windows, functions like read_socket that return -1 on
  error cannot return a size_t.

  THIS WAS A MAJOR BUG - it caused frequent, unpredictable
  crashes, and also frequently caused JB to jump to 100%
  CPU and stay there.  (Because it thought it had just
  read ((unsigned)-1) == 4Gb of data...)

- The signature of write_socket has changed, it now simply
  returns success=0/failure=nonzero.

- Trying to get rid of a few warnings --with-debug on
  Windows, I've introduced a new type "jb_socket".  This is
  used for the socket file descriptors.  On Windows, this
  is SOCKET (a typedef for unsigned).  Everywhere else, it's
  an int.  The error value can't be -1 any more, so it's
  now JB_INVALID_SOCKET (which is -1 on UNIX, and in
  Windows it maps to the #define INVALID_SOCKET.)

- The signature of bind_port has changed.