Debian package: Fix lintian warning executable-not-elf-or-script

This warning was introduced by the fix for Debian bug report #767817.

Also fix chmod call in debian/dhcpy6d.postinst (which was already
modified for the fix for Debian bug report #768989) to no more set the
executable bits on /var/log/dhcpy6d.log and
/var/lib/dhcpy6d/volatile.sqlite.

All together this fixes Debian bug report #769006.
Debian package: Replace symlink debian/dhcpy6d.logrotate with a patched copy of etc/logrotate.d/dhcpy6d

This fixes another facet of Debian bug report #769006: Removing the
executable bit from rotated logs.
Debian package: postrm: Also delete /etc/default/dhcpy6d itself, ucf doesn't
Upload to Debian unstable as 0.4-2
Merge branch 'debian-jessie'
Fix last bits of Debian bug #769006 (unnecessary executable bits)

* Remove x-bits from etc/default/dhcpy6d
* Reinstantiate debian/dhcpy6d.logrotate as symlink
Debian package: postinst script doesn't need to expect preinst parameter

For details see
https://www.debian.org/doc/debian-policy/ch-maintainerscripts.html#s-mscriptsinstact

Closes Debian bug report https://bugs.debian.org/768974
Merge branch 'master' of github.com:/HenriWahl/dhcpy6d