Allow secure-netboot

When doing file verification, tftp needs to be able to handle multiple
open files concurrently.
We also need tftp_stat() to provide useful values for st_dev and st_ino.

Allow an architecture to define NETPROTO_DEFAULT.
The default is NET_NFS for backwards compatability.

In net_parse_rootpath() fix parsing of
<scheme>://<ip>[:<port]/<path>
and ensure we return INADDR_NONE unless we successfully
parsed an addr, so we don't end up clobbering rootip obtained
from bootp().

Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D51187