Fix: add a get HOME dir fallback to getpwuid

If the $HOME or $LTTNG_HOME is not defined, fallback on getpwuid(3) of
the process uid.

Fixes #815

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: missing include for getpwuid()

Signed-off-by: David Goulet <dgoulet@efficios.com>