[SPARK-1394] Remove SIGCHLD handler in worker subprocess

It should not be the responsibility of the worker subprocess, which
does not intentionally fork, to try and cleanup child processes. Doing
so is complex and interferes with operations such as
platform.system().

If it is desirable to have tighter control over subprocesses, then
namespaces should be used and it should be the manager's resposibility
to handle cleanup.

Author: Matthew Farrellee <matt@redhat.com>

Closes #1247 from mattf/SPARK-1394 and squashes the following commits:

c36f308 [Matthew Farrellee] [SPARK-1394] Remove SIGCHLD handler in worker subprocess

(cherry picked from commit 3c104c79d24425786cec0034f269ba19cf465b31)
Signed-off-by: Aaron Davidson <aaron@databricks.com>