Fix: libc internal mutex races with run_as

Implement a proper run_as worker process scheme to fix internal libc
mutex races. Those races lead to having the internal mutex held by
another process when clone() is called, thus hanging the clone child.

Now that we create the worker process when the parent process is
still single-threaded, we don't run into those issues. Implement a
standard fork + file descriptor passing over unnamed unix sockets rather
than the prior clone + shared file descriptor table, which was causing
issues with valgrind.

This adds a new process called "lttng-runas" for each sessiond
and consumerd process.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>