vs: new functions in preparation of privsep

* initVCs
* sendPR
* chroot
* createServer
* _trackOplogVc
* _verifyAuthRequest
vcexec: make chrootConfig optional
preauth: send listen when ready
preauth: rename name of default socket
new server2.js: fork preauth and verify auth req

copy config/example.json to config/test.json

start server with sudo ./server2.js

test with something like:
echo '{ "username": "youruser", \\
"password": "yourpass", "database": "test", \\
"collection": "foo" }' | nc -w 5 127.0.0.1 2344