Improve netword FD type detection

Fix mapping between family and fdtype.
Add heuristic to set the FDtype to net if a process does a net_dev_xmit
during a write syscall. For now, we use the maybe_net type because we do
not guarantee this information to be exact.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>