Make sure forked process has stopped when deleting MythSystem

This forces a termination in a forked MythSystem process when the
MythSystem object is destroyed, preventing a situation where the private
internal object tries to access resources from the no-longer-existent
parent object.

Fixes #11412
(cherry picked from commit 9c51ad39a65eee7a3f985a127700fbc8473ffb4e)
Called methods against wrong object in destructor.

Refs #11412
(cherry picked from commit e1f14a5380886c6ddf95379341aa0f56481b7587)
Eliminate the segfault reported in tickets #10924, #11043, #11378.

Change the parent of the polling socket from the LoggerThread to the
polling context.  As a result, the polling socket is not deleted when the
LoggerThread is deleted and the destroyed signal is never triggered preventing
the call to disconnect() the deleted polling socket from the polling context.