cluster: resolve relative unix socket paths

Relative unix sockets paths were previously interpreted relative
to the master's CWD, which was inconsistent with non-cluster behavior.
A test case has been added as well.

PR-URL: https://github.com/nodejs/node/pull/16749
Fixes: https://github.com/nodejs/node/issues/16387


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>