exec plugin: Separate stdout and stderr.

exec plugin: Separate stdout and stderr.

Hi!

Digitemp sends errors to stderr, but collectd handles stdout and stderr
as the same and reports parsing errors.

My idea is to seperate stdout and stderr.

I also removed the special pipe handling checking for stdin...
because exec plugin will alway have a fd > 2 for pipes !!
(fd 0,1 and 2 (stdin, stdout, stderr) are used by collectd with /dev/null if demonized)

now strings from stderr will be reportet by ERROR

cu Peter

Signed-off-by: Florian Forster <octo@huhu.verplant.org>