Pull out route lookups from L2 output routines

Route lookups for routes of RTF_GATEWAY were done in L2 output
routines such as ether_output, but they should be done in L3
i.e., before L2 output routines. This change places the lookups
between L3 output routines (say ip_output) and the L2 output
routines.

The change is based on dyoung's patch submitted in the thread:
https://mail-index.netbsd.org/tech-net/2013/02/01/msg003847.html
You can find out detailed investigations by dyoung about the
issue in there.

Note that the change introduces a workaround for MPLS. ether_output
knew that it needs to fill the ethertype of a frame as MPLS,
based on a tag of an original route (rtentry), but now we don't
pass it to ehter_output. So we have to tell that in another way.
We use mtag to do so for now, which introduces some overhead.
We should fix it somehow in the future.

Discussed on tech-kern and tech-net.

(ozaki-r)