src: ensure that file descriptors 0-2 are valid

Check that stdin, stdout and stderr map to open file descriptors and
remap them to /dev/null if that isn't the case.  Protects against
information leaks or worse when io.js is started with closed stdio
file descriptors.

PR-URL: https://github.com/iojs/io.js/pull/875


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>