Requiring leptonica webp and giflib

This enables ctypes to find the slapos libmagic, instead of the system one

commit 21592651
Author: Tatuya Kamada <tatuya@nexedi.com>
Date:   Thu May 10 17:06:40 2012 +0900

    Make it more versatile.
    - Cache-Control:public,max-age= is also a valid configuration.
    - application/font resources do not need to care about Vary header.

Like this testSpellChecking will stop using the aspell coming from
operating system and wich does not always comes with appropriate
dictionaries

Keep accessing software-url early, in case it was originally done to fail
before trying to connect if this mandatory parameter is missing.

Those sections already exist in extended instance-zope.cfg .

We are never supposed to merge anything. We are supposed to just follow
upstream. pull merges, so it's just the wrong tool.
Instead, fetch and reset.
It is not a problem to always reset, as user already cannot rely on
repository's survival across updates: if there is any error reported by
git command, the repository will be wiped out. So resetting will not do
any more harm than what can already happen.
Also, properly quote paths to protect against spaces (in both command and
update-command).

reason:
 - Expires header is widely supported than max-age. e.g.: HTTP/1.0 clients.
 - max-age directive always overrides Expires setting under HTTP/1.1,
   so there is no need to remove Expires here.

so that varnish recipe can be used without webchecker.

(cherry picked from commit c060818b385d8af429725fb9c53fdb8068ad21d5)

(cherry picked from commit 7d8d21d815e6a99ba5959d28b7d59c69d034e8c3)

6.7.6-7 fixes issue of darkening some images.
(cherry picked from commit 62885346a1b66d7d7d94b682c37316ad0b622a6a)

(cherry picked from commit 22b4543965c38a5b6c78730240c3dbfdd8079781)

The librsvg contains rsvg-convert, the command used by imagemagic
to convert svg to png.

This is a cosmetic modification. Functionally, there is no change.

"Introduce the best practices from erp5 public web sites experience."
1ed2b682

This commit only includes functional modification.