* Unify headers already sent errors
Now whenever we need to check where headers were already sent in
ext/session, we call a single location that prints where, keeping it
consistent output wise.
* Unify session aready started errors
Similar to the one for headers.
* Also change session active checks too
This usually go hand in hand with the headers already sent checks, but
is in a separate commit because of the amount of tests it changes.
These hopefully test a reasonable set of basic, error and variations for
the twenty or so session functions. Note however that they do not
test all the session configuration settings.
