RFC: Show the PID of each worker when running under valgrind

[garrison]

When valgrind outputs any warning/error messages, it does so along with a banner of the getpid() that the error is coming from. This is immensely helpful when working to debug programs that fork multiple processes.

This patch causes each such julia worker process to output the getpid() of the worker when the worker starts (but only when running under valgrind). This allows a person to determine which worker number each error/warning comes from. For instance, given the following (schematic) output, it is now possible to determine that the "invalid read" warning is due to the linalg1 tests.

$ valgrind -q --smc-check=all-non-file --trace-children=yes --suppressions=$PWD/../contrib/valgrind-julia.supp ../julia runtests.jl linalg
    From worker 2:  PID = 12609
    From worker 3:  PID = 12610
==12609== Invalid read of size 16
[snip]
From worker 3:       * linalg2
From worker 3:       * linalg3
From worker 2:       * linalg1

I put this patch where I thought it would fit, but I'd be happy to move it if there's a better place.

[timholy]

LGTM.