pciconf: Add a tree mode

[jhb]

Summary

This lists PCI devices in a hierarchy showing the parent/child
relationship of PCI devices and bridges. While this is inspired by
lspci -t output, the format is closer to ps -d and also prefers using
new-bus device names when possible. If a device does not have a
driver, the PCI selector is output in place of the device name.

[jhb]

Sample output available at https://reviews.freebsd.org/P704

[imp]

I love this