cluster: support options in Worker constructor

This commit moves some common Worker code into the constructor
via support for an options argument.

Reviewed-By: Fedor Indutny <fedor@indutny.com>
tests: improve test-child-process-fork-dgram.

Fixes test-child-process-fork-dgram on the SmartOS jenkins node.

1) Wait for child process to register event handlers on server UDP socket
before sending messages from the parent process.

2) Increase the number of total messages so that both the child and the
master process get a chance to handle at least one of them.