Improve reliability of downloads from custom node mirrors

[shadowspawn]

Technical. Opening this partly so I can close some related old Pull Requests!

There are occasionally problems reported by users in China using https://npm.taobao.org/mirrors/node for downloads. These are partly caused by differences from the main mirror encountered during site scraping to find version numbers, partly due to extra redirects, and partly due to faulty code introduced trying to fix those...

Tentative plan:

• rename environment variable to N_NODE_MIRROR and publicise in README
• use command status codes and support redirects instead of reading command output to detect redirects
• use index.tab instead of screen scraping

[shadowspawn]

Prerelease available via npm i n@next for v6. Release notes: https://github.com/tj/n/releases/tag/6.0.0-0

[shadowspawn]

Released: https://github.com/tj/n/releases/tag/v6.0.0