Clean-up: Presume that Linux is always IPv6 capable at build time

Clean-up: Presume that Linux is always IPv6 capable at build time

These days it is highly unlikely that OpenVPN will be built in a non-IPv6
capable Linux environment.  So remove compile-time related macros identifying
that.

This also solves an issue which was introduced in commit 51bd56f46f55177cf0f8b
where HAVE_TUN_PI is no longer detected.  The tun_pi struct is defined in
linux/if_tun.h, which will be checked for later on.  As this struct has history
in linux/if_tun.h all back to the beginning of the kernel git tree (2.6.12-rc2,
April 2005), it is considered not needed to check for this struct explicit.

[ v2: Commit 7c0a2b5f2b4409 modifies some of the checks this patch touches.  This
      patch just adopts to those changes ]

Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Alon Bar-Lev <alon.barlev@gmail.com>
Message-Id: 1335521425-23391-1-git-send-email-davids@redhat.com
URL: http://article.gmane.org/gmane.network.openvpn.devel/6351