Introduce LTTNG_UST_CTL_PATH environment variable

Introduce LTTNG_UST_CTL_PATH environment variable

Example use to trace lttng-sessiond (tracee) with another lttng-sessiond
(tracer):

```
mkdir /tmp/tracer-ust
mkdir /tmp/tracee-ust
mkdir /tmp/tracer-home
mkdir /tmp/tracee-home
LTTNG_HOME=/tmp/tracer-home LTTNG_UST_CTL_PATH=/tmp/tracer-ust lttng-sessiond --daemon
LTTNG_HOME=/tmp/tracee-home LTTNG_UST_CTL_PATH=/tmp/tracee-ust \
	LTTNG_UST_APP_PATH=/tmp/tracer-ust \
	LD_PRELOAD=/usr/local/lib/liblttng-ust-fork.so:/usr/local/lib/liblttng-ust-fd.so \
	lttng-sessiond --daemon
```

* Control the tracer sessiond by setting LTTNG_HOME=/tmp/tracer-home
* Control the tracee sessiond by setting LTTNG_HOME=/tmp/tracee-home
* Run an application traced by the tracee sessiond

```
LTTNG_UST_APP_PATH=/tmp/tracee-ust myapp
```

* Run an application traced by the tracer sessiond

```
LTTNG_UST_APP_PATH=/tmp/tracer-ust myapp
```

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ifcd05c366004b5fc0a4a362ad3556d7e6d8658b1