Constant strings and logic for HTTP(S) default ports were starting to be
spread throughout netops.c.  Instead of duplicating this again to
determine if a Host header should include the port, move the default
port constants and logic into an internal method in netops.{c,h}.

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.

This is what it's meant all along, but now we actually have multiple
implementations, it's clearer to use the name of the library.

Most of the network-facing facilities have been copied to the socket and
openssl streams. No code now uses these functions directly anymore, so
we can now remove them.

[factor] Join typedef and struct definitions in single file.

This option make it easy to ignore anything about the server we're
connecting to, which is bad security practice. This was necessary as we
didn't use to expose detailed information about the certificate, but now
that we do, we should get rid of this.

If the user wants to ignore everything, they can still provide a
callback which ignores all the information passed.

OpenSSL's tests init everything in the main thread, so let's do that.

This kind of stuff should have unit tests, even if it's just to show
what we expect to match successfully.