nd6 defrouter: consolidate nd_defrouter manipulations in nd6_rtr.c

Move the nd_defrouter along with the sysctl handler from nd6.c to
nd6_rtr.c and make the variable file static. Provide (temporary)
new accessor functions for code manipulating nd_defrouter from nd6.c,
and stop exporting functions no longer needed outside nd6_rtr.c.
This also shuffles a few functions around in nd6_rtr.c without
functional changes.

Given all nd_defrouter logic is now in one place we can tidy up the
code, locking and, and other open items.

MFC after: 3 weeks
X-MFC: keep exporting the functions
Sponsored by: Netflix