Add rcorder-visualize.sh, which generates graphviz from rc.d scripts

This is imported from NetBSD. The author--Joerg Sonnenberger--agreed
to apply a two-clause BSD license, just so the license was clear.

This source tree location matches NetBSD, and is the first place someone
might look for such a tool.

Obtained from: Joerg Sonnenberger via NetBSD
MFC after: 3 days
Sponsored by: Dell EMC