Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.

This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.

This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.

ref:
https://github.com/gentoo/libressl/blob/672ac74ce7b7cb2e4799b2d66bc0b1b1efa3454e/media-video/ffmpeg/files/ffmpeg-3.2-libressl.patch

We need to include the initialisation and construction functions in all
backend, so we include this header when building against SecureTransport
and WinHTTP as well.

For older versions we can fall back on the deprecated ASN1_STRING_data.

We want to program against the interface, so recreate it when we compile
against pre-1.1 versions.

This unfortunately isn't as stackable as could be possible, as it
hard-codes the socket stream. This is because the method of using a
custom openssl BIO is not clear, and we do not need this for now. We can
still bring this in if and as we need it.