swc-installation-test-2.py: Isolate system-info indent in _print_info

This way shifting the indents if you add a longer key is a single-line
operation.
swc-installation-test-2.py: Use the more portable platform.uname()

From the docs [1,2]:

  platform.uname():
    Fairly portable uname interface...  Entries which cannot be
    determined are set to ''.

  os.uname():
    ... Availability: recent flavors of Unix.

[1]: http://docs.python.org/2/library/platform.html#platform.uname
[2]: http://docs.python.org/2/library/os.html#os.uname
swc-installation-test-2.py: Add OS-version detection

Using assorted functions from the `platform` module.  On my Linux box:

  $ python2.7
  Python 2.7.3 (default, Dec  5 2012, 10:56:01)
  [GCC 4.5.4] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import platform
  >>> platform.linux_distribution()
  ('Gentoo Base System', '2.1', '')
  >>> platform.mac_ver()
  ('', ('', '', ''), '')
  >>> platform.win32_ver()
  ('', '', '', '')

On an OS X box:

  $ python2.6
  Python 2.6.1 (r261:67515, Aug  2 2010, 20:10:18)
  [GCC 4.2.1 (Apple Inc. build 5646)] on darwin
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import platform
  >>> platform.linux_distribution()
  ('', '', '')
  >>> platform.mac_ver()
  ('10.6.8', ('', '', ''), 'i386')
  >>> platform.win32_ver()
  ('', '', '', '')

I don't have access to an MS Windows box, but presumably it returns
something like:

  >>> platform.linux_distribution()
  ('', '', '')
  >>> platform.mac_ver()
  ('', ('', '', ''), '')
  >>> platform.win32_ver()
  ('XP', '5.1.2600', 'SP2', 'Multiprocessor Free')

We only want to print the one that applies (e.g. don't print a
'linux_distribution' entry on OS X), so we skip entries where value[0]
is an empty string.