Abstract daemon file descriptor setup

This does some slight changes:

- Daemonize() doesn't explicitly close the file-descriptors anymore, but
  only implicitly via the usage of dup2
- StartDaemonChild uses separate devnull for stdin (rdonly) and
  stdout/stderr (wronly), or if using a log file, it uses it in append
  mode

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>