Cleaned up and updated INSTALL

Cleaned up and updated INSTALL

- Changed "Supported platforms" to reflect current support status of the
  mentioned operatingsystems
- Removed mentions of the obsolete Python and domake-win buildsystems
- Added mention of official Debian/RPM packages
- Added links to the Wiki
- Added mention of the new openvpn-build cross-compile environment
- Added PolarSSL to and removed pthreads from the "Optional" section
- Added mention of t_client.sh test framework
- Removed some very old (pre-2005) nuggets of (obsolete) information
- Other minor cleanups
- Some reorganization

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: 1359107974-1481-1-git-send-email-samuli@openvpn.net
URL: http://article.gmane.org/gmane.network.openvpn.devel/7303
Signed-off-by: Gert Doering <gert@greenie.muc.de>