Check for dbus-daemon in /usr/bin

`enter-chroot` launches a chroot-specific dbus instance, and checks if `dbus-daemon` is installed by seeing if `/bin/dbus-daemon` is executable. But Debian keeps its `dbus-daemon` at `/usr/bin/dbus-daemon`, which means dbus isn't starting automatically on `enter-chroot`.

This commit fixes that by checking for either `/bin/dbus-daemon` or `/usr/bin/dbus-daemon` in the chroot.

Refs #279