src: ignore risky env vars when setuid root

On POSIX platforms, check that the uid and gid match the euid and egid
respectively before looking up the environment variable.

Before this commit, an i18n-enabled suid node would cheerfully load
attacker-controlled ICU data through the NODE_ICU_DATA environment
variable.

This commit is not a complete fix.  For example, it's up for debate
what to do with the NODE_CHANNEL_FD environment variable.

PR-URL: https://github.com/node-forward/node/pull/18


Reviewed-By: Fedor Indutny <fedor@indutny.com>