Add ip route save/restore

Add ip route save/restore

This patch adds save and restore commands to "ip route". Save dumps
the RTNL stream to stdout which can then be passed to restore later.
This may be helpful in some normal situations, and will allow C/R to
migrate the routing information in userspace.  Tweaking of the stream
can be done by userspace helpers to convert between versions and adjust
things like device indexes when restoring routes in a different
environment.

By factoring out some of the common bits of print_route() into
filter_nlmsg(), the "save" command can use the same selection logic
as "list," allowing the caller to save only specific routes as
necessary.

The only change since the RFC is the addition of manpage and doc
material.

Signed-off-by: Dan Smith <danms@us.ibm.com>