Fixed ProcTest.MultipleThreads test.

Review: https://reviews.apache.org/r/19952
Adds a "setup" function to process::subprocess.

Adds the ability run a "setup" function to process::subprocess within
the forked child context, after fork and before exec.

NOTE: Such lambda must not contain any async unsafe code. For details
on async safety, see POSIX.1-2004 on async-signal-safe functions, also
referenced in the signal man-pages:
http://man7.org/linux/man-pages/man7/signal.7.html.

Review: https://reviews.apache.org/r/19259