netdev: Factor restoring flags into new "struct netdev_saved_flags".

This gets rid of the only per-instance data in "struct netdev", which
will make it possible to merge "struct netdev_dev" into "struct netdev" in
a later commit.

Ed Maste wrote the netdev-bsd changes in this commit.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Co-authored-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Ed Maste <emaste@freebsd.org>
Tested-by: Ed Maste <emaste@freebsd.org>