Unset LD_PRELOAD after initialisation to prevent propagation to children

If the process being monitored spawns children, ensure that the module
isn't loaded by them, by unsetting the LD_PRELOAD environment variable.
This is particularly important when using python-dbusmock, as
Python/GLib interacts badly with libgobject-list.so.

This can be disabled by setting the GOBJECT_PROPAGATE_LD_PRELOAD
environment variable to any value.
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