Refactor BasedirOptions to improve accuracy of help messages.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Some minor refactorings for clearer code.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Improve messages from tahoe start/run.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Improve a comment to take Windows into account.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>