Switch IPv4 output path to use new routing api.

The goals of the new API is to provide consumers with minimal
  needed information, but as fast as possible. So we provide
  full nexthop info copied into alighed on-cache structure
  instead of rte/ia pointers, their refcounts and locks.
  This does not provide solution for protecting from egress
  ifp destruction, but does not make it any worse.

Current changes:

nhops:
Add fib4_lookup_prepend() function which stores either full
L2+L3 prepend info (e.g. MAC header in case of plain IPv4) or
L3 info with NH_FLAGS_L2_INCOMPLETE flag indicating that no valid L2
info exists and we have to take "slow" path.

ip_output:
Currently ip[ 46]_output consumers use 'struct route' for
the following purposes:
  1) double lookup avoidance(route caching)
  2) plain route caching
  3) get path MTU to be able to notify source.
The former pattern is mostly used by various tunnels
 (gif, gre, stf). (Actually, gre is the only remaining,
 others were already converted. Their locking model did
 not scale good enogh to benefit from such caching, so
 we have (temporarily) removed it without any performance
 loss).
Plain route caching used by SCTP is simply wrong and should be removed.
  Temporary break it for now just to be able to compile.
Optimize path mtu reporting by providing it in new 'route_info' stucture.

Minimize games with @ia locking/refcounting for route lookup:
  add special nhop[46]_extended structure to store more route attributes.
  Pointer to given structure can be passed to fib4_lookup_prepend() to indicate
  we want this info (we actually needs it for UDP and raw IP).

ether_output:
Provide light-weight ether_output2() call to deal with
transmitting L2 frame (e.g. properly handle broadcast/simloop/bridge/
  other L2 hooks before actually transmitting frame by if_transmit()).
Add a hack based on new RT_NHOP ro_flag to distinguish which version should
  we call. Better way is probably to add a new "if_output_frame" driver
  callbacks.

 Next steps:
* Convert ip_fastfwd part
* Implement auto-growing array for per-radix nexthops
* Implement LLE tracking for nexthop calculations to be able to
  immediately provide all necessary info in single route lookup
  for gateway routes
* Switch radix locking scheme to runtime/cfg lock
* Implement multipath support for rtsock
* Implement "tracked nexthops" for tunnels (e.g. _proper_
  nexthop caching)
* Add IPv6 support for remaining parts (postponed not to
   interfere with user/ae/inet6 branch)
* Consider adding "if_output_frame" driver call to
  ease logical frame pushing.