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.

giterr_set: consistent error messages

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

We want to keep the git UA in order for services to recognise that we're
a Git client and not a browser. But in order to stop dumb HTTP some
services have blocked UAs that claim to be pre-1.6.6 git.

Thread these needles by using the "git/2.0" prefix which is still close
enough to git's yet distinct enough that you can tell it's us.

When calling `http_connect` on a subtransport whose stream is already
connected, we first close the stream in case no keep-alive is in use.
When doing so, we do not reset the transport's connection state,
though. Usually, this will do no harm in case the subsequent connect
will succeed. But when the connection fails we are left with a
substransport which is tagged as connected but which has no valid
stream attached.

Fix the issue by resetting the subtransport's connected-state when
closing its stream in `http_connect`.