src: add NODE_PRESERVE_SYMLINKS environment variable

Add a way through environment variables to set the --preserve-symlinks
flag. Any non-null value of NODE_PRESERVE_SYMLINKS will enable symlinks.

PR-URL: https://github.com/nodejs/node/pull/8749
Fixes: https://github.com/nodejs/node/issues/8509


Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>