posix: Add in6 platform headers

Upstream Click is now resorting to platform-wide headers
for in6_addr. lwip 1.3 doesn't have these, so we extract it from BSD
temporarily.

In future we will migrate towards lwip 1.4.1 which already contains
IPv6 support.
pvh: initial patch of pvh port to mini-os for clickos

This patch is rebased to mini-os for clickos by using the commit of
1c32dd9f7a3ed in https://github.com/hwanju/xen (minios-pvh branch).
This initial patch doesn't handle new features of mini-os for clickos
like suspend/resume.
pvh: Added missing xen-features.h

This file refers to commit 1c32dd9f7a3ed in
https://github.com/hwanju/xen (minios-pvh branch)