Copy system cert configs when installing new versions

[squarebracket]

This PR adds support for copying any ca, cert, or cafile configs from the system's npm config file to the newly-installed npm prefix's config. It does this by default, but can be disabled with the --skip-system-certs flag.

Manually tested in sh, bash, and zsh. Also tested in ksh (per CONTRIBUTING.md), but that doesn't seem to work unrelated to my changes.

Any suggestions on how I could approach automated testing for these changes? Specifically, how I would mock out the nvm system exec npm config ls call.