FreeBSD: generate devd.conf files for USB UPSes

This adds a --with-devd-dir=PATH option to ./configure, which defaults to
/usr/local/etc/devd (or /etc/devd, whichever is found first).

Unlike udev, there does not seem to be a way to re-trigger rules at runtime.
This means you will likely need to unplug and replug your UPS after installing
the new nut-usb.conf file.