enter-chroot: Autodetect dbus user and group.

I leverage on `xmllint` that is installed in Chrome OS to read dbus configuration file, and then use `id` to make sure the user exists, and to figure out its default group.

`xmlint` is not really meant to be used in scripts, so the shell cannot be made quiet: it outputs prompts and other characters, but since we know what a good user name looks like, we can grep it out.

I also have a nice XSLT solution that is a big longer, but somewhat cleaner. Unfortunately, no XSLT processor on Chrome OS (and we do not want to install it on the chroot just for this purpose...)