Handle multiple version numbers when having more than one linux flavours for splash.svg post-processing.

Removing spurious lb_source_netboot, there's not such thing as a netboot specific source tarball anyway.

Adding support for zsync control files generation, thanks to Hector Oron <hector.oron@gmail.com> for the idea and prototype.

Applying patch from Steven Shiau <steven@nchc.org.tw> to conditionally copy /etc/apt/secring.gpg* files.

Truncating udev persistent net rules in remove-udev-persistent-rules.chroot hook rather than to remove them (Closes: #680763).

This allows to have non-persistent net devices with persistence properly,
see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=680763#10 for more information.

Changing default binary.list filename to binary.contents to make it more explicit and not to be confused with package lists.

For consistency, respecting config/environment too for both chroot and binary environments (but the content of sepcific environments for either chroot or binary still has precedence over the generic one).