sd-bus: the bus returned should be the first arg

This matches the API of previous headers, such as sd-journal.h.
api: in constructor function calls, always put the returned object pointer first (or second)

Previously the returned object of constructor functions where sometimes
returned as last, sometimes as first and sometimes as second parameter.
Let's clean this up a bit. Here are the new rules:

1. The object the new object is derived from is put first, if there is any

2. The object we are creating will be returned in the next arguments

3. This is followed by any additional arguments

Rationale:

For functions that operate on an object we always put that object first.
Constructors should probably not be too different in this regard. Also,
if the additional parameters might want to use varargs which suggests to
put them last.

Note that this new scheme only applies to constructor functions, not to
all other functions. We do give a lot of freedom for those.

Note that this commit only changes the order of the new functions we
added, for old ones we accept the wrong order and leave it like that.
update TODO