mwan3: propagate mwan3 use command exit code

mwan3: propagate mwan3 use command exit code

This propagates the exit code of the command wrapped by `mwan3 use` and
allows for example to use `mwan3 use` in monitoring scripts.

Before change:
shell command:
  mwan3 use wan false >/dev/null && echo ok || echo fail
result:
  ok

After change:
shell command:
  mwan3 use wan false >/dev/null && echo ok || echo fail
result:
  fail

Signed-off-by: Philipp Schmitt <philipp@schmitt.co>