Merge branch 'nexthop-objects' into next

David Ahern  says:

====================

This set adds support for nexthop objects to the ip command. The syntax
for nexthop objects is identical to the current 'ip route .. nexthop ...'
syntax making it easy to convert existing use cases.

v2
- Fixed header use in rtnl_nexthopdump_req as noted by roopa
- made rth_del static per Stephen's request and fixed coding style
- removed print_nh_gateway and exported print_rta_gateway to reuse
  the iproute.c code (keeps consistency in output)
- added examples to commit message
- fixed monitor use when specific groups requested
- fixed usage in 'ip nexthop'
- added manpage

====================

Signed-off-by: David Ahern <dsahern@gmail.com>