Add tuntap_get_fd() and deprecate TUNTAP_GET_FD
Add a note about tuntap_set_descr() compat
Slightly change an error message

This avoid a confusion about which function is the source of the error message.
Document tuntap_del_ip
Check for features, not OSes!

Replace #ifdef goo for a lot of headers inclusion:
HAVE_NETINET_ETHER_H vs HAVE_NETINET_IF_ETHER_H
HAVE_NET_IF_TUN_H vs HAVE_LINUX_IF_TUN_H vs HAVE_NET_TUN_IF_TUN_H
HAVE_NET_IF_H vs HAVE_LINUX_IF_H

It's the responsability of CMake (or any other build systems) to
properly define these definitions.
For the moment it is statically defined in CMakeLists.txt.
Add CMakeLists.txt.local

This file contains commented out suggestions for some specific use case,
like specifying the -std=c++11 flags or working on Solaris without Sun
Compilers Studio.
Add a `how to build` in the README
Yet another tuntap.h diet!

Prevent some others system headers from beeing pulled and move
some internal definition, which are not needed for the public library
header.

This break further the Windows port, but I'm prety shure it was not
working for months.