Four new features and a bugfix.

Four new features and a bugfix.

- Center the clock on the user's terminal.
- Check that the terminal is sufficiently large to fully display the
  clock (about 61x9.)  If not, exit immediately with an error.
- Introduce a short delay in the scrolling when -s is given, so that
  it can be better appreciated on syscons(4) and local xterm(1).  The
  default delay is 120 milliseconds.
- Add a new option, -d, to allow changing the scroll delay to any
  duration from 0 to 5000 milliseconds.  The -d option implies -s.
- Make it so that, when the optional argument is omitted, the clock
  really does run forever.  (Before this, it would have stopped after
  about 65536 seconds due to wraparound.)