Fix: make lttng expand path for trace output opt

lttng create --output was passing the path string to the session daemon
and thus, for relative path like './mytraces', it was created in the
current directory of the session daemon.

Now lttng command line uses the realpath(3) of the --output string and
denies creation if multiple level of directory does not exist (Ex:
/tmp/foo/bar/chap, if foo/ does not exist, it is refused).

Directory creation still occurs on the session daemon side.

Reported-by: Ettore Del Negro <ettore@ettoredelnegro.me>
Signed-off-by: David Goulet <dgoulet@efficios.com>