Move sending of router solicitations to the kernel; receiving and

Move sending of router solicitations to the kernel; receiving and
processing of router advertisements was already in the kernel.
With this rtsol{,d}(8) is no longer necessary.

The kernel starts sending solicitations with
# ifconfig $IF inet6 autoconf
or
inet6 autoconf
in /etc/hostname.$IF.

input stsp@
much help & OK mpi@
tweaks & OK bluhm@