Fix: leak of sessiond configuration on launch of run-as worker

The run-as worker is spawned through fork() without using
exec*(). This means that any resource allocated by the session
daemon before the launch of the run-as worker will be leaked in
the run-as worker's process.

A callback is added to the run_as launch interface to allow users
a chance to clean-up after the fork occurs. This mechanism is
fragile as it may not always be easy (or possible) to track all
such resources in the future. This makes a strong argument for using a
new process image (through exec*()) and forego any such problem at
some point.

The lttng-consumerd from a similar (and more severe) problem with its
own run-as worker. A fix adressing the consumerd's problem follows.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>