Significantly simplifies the xroute logic.

Run make EXTRA_DEFINES=-DNO_LOCAL_INTERFACE.

It is now controlled by -g.

This doesn't matter for regular Babel, but it breaks the babel-local-interface
branch, which uses a TCP socket for communicating with the GUI.

No hello is -1, not 0.

We now remember the number of kernel routes from one call of check_xroutes
to the next to avoid calling down into the kernel multiple times.