src: do platform-specific initialization earlier

Execute the per-platform initialization logic as early as possible,
for two reasons:

1. It opens the way for an upcoming commit to simplify early SIGUSR1
   handling.

2. It should make life easier for embedders because io.js no longer
   mucks around with the file descriptor limit or signal disposition
   of the process.

PR-URL: https://github.com/iojs/io.js/pull/615


Reviewed-By: Sam Roberts <sam@strongloop.com>