mdadm: Unify forks behaviour

mdadm: Unify forks behaviour

If mdadm is run by udev or systemd, it gets a pipe as each stream.
Forks in the background may run after an event or service has been
processed when udev is detached from pipe. As a result process
fails quietly if any message is written.
To prevent from it, each fork has to close all parent streams. Leave
stderr and stdout opened only for debug purposes.
Unify it across all forks. Introduce other descriptors detection by
scanning /proc/self/fd directory. Add generic method for
managing systemd services.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>