Debian package: a more sophisticated postrm

Ideas and code taken from ucf's postrm example
Debian package: Install volatile.sqlite into /usr/share/dhcpy6d/

Copy it to /var/lib/dhcpy6d/volatile.sqlite during postinst only if it
doesn't yet exist. Remove /var/lib/dhcpy6d/volatile.sqlite again upon
package purge.

Closes Debian bug report https://bugs.debian.org/768989