Always open index page with serve --open

[ISSOtm]

This fixes an issue where mdbook serve --open may land on a 404 page if the file name is changed. This is easily reproduced by running mdbook serve --open guide on mdBook itself, which will open http://localhost:3000/README.html, which 404's thanks to the index preproc.

Since mdBook seems to always generate an index.html page for the first chapter anyway, it seems that "trimming the fat" is also more correct.

[ehuss]

Thanks! This was intentionally changed in #1714 to avoid problems with draft chapters. Can you find a solution that fixes both issues?

[ISSOtm]

I'd say this is fixed by #1829, which ensures that index.html is always generated, even if there are draft chapters before the first "real" one. (If there are no non-draft chapters, this will 404, but there's nothing to serve, anyway.)

[ehuss]

Can you also update build.rs and watch.rs?

[ISSOtm]

What needs to be changed?

[ehuss]

They both have --open flags with code that needs to be updated. More or less, this should revert #1714.

[ISSOtm]

Done!

[ehuss]

Thanks!