If you call lldpcli from a program, stdout will be fully buffered, and
we won't get back the result of "show neighbors" (nor anything else).
Expectedly, stderr messages are delivered immediately.

You can verify this by running "cat | lldpcli", give comamnds and
then exit with ctrl-D (EOF).

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>