Introduce LTTNG_UST_APP_PATH environment variable

Introduce an environment to specify a path under which unix sockets
used for the communication between the application (tracee) instrumented
with `liblttng-ust` and the LTTng session and consumer daemons (part of
the LTTng-tools project) are located. When `$LTTNG_UST_APP_PATH` is
specified, only this path is considered for connecting to a session
daemon. Setting this environment variable disables connection to root
and per-user session daemons.

The `$LTTNG_UST_APP_PATH` target directory must exist and be accessible
by the user before the application is executed for tracing to work.

This environment variable affects the Java and Python agents in the same
way.

This environment variable on the LTTng-UST application side is meant to
be used with a new LTTNG_UST_CTL_PATH on the lttng sessiond side.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I4784f4565514a9771827603bd0bebabbeb37a7ad