the ROUTETABLES kernel option thus there is no need to include opt_route.h
anymore in all consumers of vnet.h and no longer depend on it for module
builds.
Remove the hidden include in flowtable.h as well and leave the two
explicit #includes in ip_input.c and ip_output.c.
adjust conf/files and modules' Makefiles accordingly.
No code or ABI changes so this and most of previous related
changes can be easily MFC'ed
MFC after: 5 days