Fix spurious timeout message and curdir. Closes #70.

Commit 69afbc5 caused some problems.

Also:

- Fix the timeout logic to use `accept-process-output' and a deadline.

- Increase the timeout default to 30 seconds.

- Always wait for the prompt.

- Display some informative messages while starting Racket and waiting
  for the prompt.

- Improve some comments, doc strings, and variable names.