cluster, v8: fix --logfile=%p.log

The %p is replaced with the current PID.  This used to work in node.js
v0.9.7 but it seems to have been lost somewhere along the way.

This commit makes the fix from 6b713b52 ("cluster: make --prof work for
workers") work again.  Without it, all log data ends up in a single
file and is unusable because the addresses are all wrong.

Suggested reviewer: @tjfontaine