downloadunpacked: Because shared part should not be uninstalled, install method should return an empty list in shared mode.

download, downloadunpacked: Rename 'shared-part' to 'shared-parts'. Since the original option name is 'parts', 'shared-parts' is consistent.

download, downloadunpacked: Disable shared feature by default since it is stil an experimental one. Enable it only if shared-part is defined.

This recipe alters a system perl ( or perl installed by another buildout
recipe ), which is not consistent with the independence of parts we are
trying to achieve in order to be able to cache and share parts.

SlapOS comonents now expose a `perl-CPAN-package` macro that should be
used for consistency. See nexedi/slapos!341

/reviewed-on nexedi/slapos.recipe.build!4

For Stretch, the installer needs more than 256M.

setuptools.archive_util is not available unless setuptools.archive_util
have been imported before.

For example:
```python
>>> import setuptools
>>> setuptools.archive_util
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'archive_util'
>>> import setuptools.archive_util
>>> setuptools.archive_util
<module 'setuptools.archive_util' from '/usr/local/lib/python2.7/dist-packages/setuptools/archive_util.pyc'>
```

/reviewed-on !3

gitclone: Fix upload_network_cached. helper_upload_network_cached_from_directory needs two additional parameters since slapos.libnetworkcache 0.14.5.

Since slapos.libnetworkcache needs configuration, the user already
pulls it explicitely. Do not annoy those who don't use it.

In our case, it breaks offline builds with Python 2.6 when the
download-cache is prepared with Python 2.7: argparse missing.

zc.buildout.download.Download may return temporary files
(only readable/writable by the current user) and by default
we'd like installed files to respect umask.

In the case of SlapOS, we usually want installed files to be readable
by members of 'slapsoft' group.

Not fully tested. With this commit, it is at least possible to
bootstrap SlapOS from Python 3.