Set the exit status of ustctl main function v3

Each possible command checks the return value and, on error,
print on stderr and set the retval. This makes ustctl return
an EXIT_FAILURE on any command that fails.

Signed-off-by: David Goulet <david.goulet@polymtl.ca>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>