Starting from NumPy 1.10 numpy's distutils support parallel building

    http://docs.scipy.org/doc/numpy/user/install.html#basic-installation
    https://github.com/numpy/numpy/commit/23d54617

and this way software which uses numpy's distutils (scipy,
scikit-learn, etc) should support it too.

Let's use it, like we currently already use MAKEFLAGS for speeding up
make-based projects.

/cc @jerome, @cedric.leninivin, @kazuhiko

Currently we use '%d' and string formatting on max(1, ncpu / cpu-usage-ratio),
because `ncpu / cpu-usage-ratio` is float:

    In [1]: from __future__ import division

    In [2]: 8 / 4
    Out[2]: 2.0

and jinja2 uses future division by default:

    {{ 8 / 4 }}   ->   2.0

We can however make things more explicit, by explicitly using integer
division (// operator) and this way avoid the need for '%d' and string
formatting.

/cc @jerome, @cedric.leninivin

The calculation is full line long, and we are going to reuse this number
in a couple of new places, so this way it makes sense to first compute
jobs number separately, and then reuse the variable.

/cc @jerome, @cedric.leninivin

/reviewed-by @kazuhiko  (on !21)

Commit 6f009a9f broke generation of zope-*.conf

Partition root should be used to control overall permissions, which is
typically a safe-enough 0750 (depending on slapformat and sysadmin).
umask is applied at libc mkdir level, so this typically results in 0755
actual permission.
More restrictive permissions should only be used when really needed.

/reviewed-by @kazuhiko

This is a follow-up of commit 6f009a9f

Conflicts:
	stack/slapos.cfg

NEO is still in heavy development and it must be easy to update code on
existing deployment.

Zope integrated Products.signalstack back in 2010:

    https://github.com/zopefoundation/Zope/commit/16796274
    ( Integrated the Products.signalstack / z3c.deadlockdebugger
      packages. You can now send a SIGUSR1 signal to a Zope process and get a
      stack trace of all threads printed out on the console. This works even
      if all threads are stuck. )

In 404150b7 (wendelin: Teach -dev build to dump all threads traceback on
SIGUSR1) it was my mistake to add Products.signalstack.

/reviewed-by TrustMe
/cc @Tyagov

This fixes the following random failures:

While:
  Installing slapos-deps-eggs.
  Getting distribution for 'ZODB3==3.10.5+SlapOSPatched001'.

An internal error occurred due to a bug in either zc.buildout or in a
recipe being used:
Traceback (most recent call last):
  ...
  File "zc/buildout/easy_install.py", line 968, in _get_dist
    shutil.copytree(dist.location, newloc)
  File "shutil.py", line 177, in copytree
    os.makedirs(dst)
  File "os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 17] File exists: '.../eggs/ZODB3-3.10.5-py2.7-linux-x86_64.egg'