cluster: make --prof work for workers

Profiling in clustered environments doesn't work out of the box.

By default, V8 writes the profile data of all processes to a single
v8.log.

Running that log file through a tick processor produces bogus numbers
because many events won't match up with the recorded memory mappings
and you end up with graphs where 80+% of ticks is unaccounted for.

Fixing the tick processor to deal with multi-process output is not very
useful because the processes may be running wildly disparate workloads.

That's why we fix up the command line arguments to include
a "--logfile=v8-%p.log" argument (where %p is expanded to the PID)
unless it already contains a --logfile argument.

Fixes #4617.