Add support for LTTNG_UST_HOME

Add support for LTTNG_UST_HOME

Namespacing the LTTNG_HOME env variable facilitates the work carried to
have a way to trace the tracer (lttng-sessiond). This also fits with
the work done lately to namespace lttng-ust.

The LTTNG_HOME environment variable is used by lttng-sessiond to setup
the whole tracing environment for the application to be traced. When
lttng-ust is loaded by the lttng-sessiond to be traced, the fact that it
reuse the `LTTNG_HOME` set for the lttng-sessiond prevent us from
specifying an external lttng-sessiond home.

Albeit it could be possible for the lttng-sessiond to "trace" itself
(self tracing), it make more sense, in our testing environment, to have
a supplementary lttng-sessiond handling the tracing of the
lttng-sessiond under testing.

Note that some work will be carried to limit the use of LTTNG_HOME to
setup the tracing environment by lttng-sessiond and liblttng-ctl APIs
but it will be a long effort. Providing `LTTNG_UST_HOME` allows us to
start dogfooding today.

`LTTNG_HOME` is still used as a fallback to `LTTNG_UST_HOME` to preserve
backward compatibility.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I6aed21fd70d1b79b6768d237f59cc80612938d65