Daemon: use double fork for daemonization

The daemon code used just a single fork to detach lnst-slave and become
a daemon. However this was insufficient and caused hangs when daemonized
inside Beaker jobs. The proper way is to use double fork method.

Version 2:

Added missing file descriptor redirection.

Signed-off-by: Jan Tluka <jtluka@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>