INI_ERR_INVALID_FILENAME is misleading

[Reissner]

In fact, the INI_ERR_INVALID_FILENAME is always when the stream cannot be opened.
That the filename is invalid is only one of the possible reasons.
This constant may be called INI_ERR_CANNOT_OPEN_STREAM_ON_FILE or something.

By the way, in my opinion, an enum would be much better.
Could be called sth like RWsucc or what.

Then of course, INI_ERR is superfluous
and we shall add sth like SUCCESS = 0.

Allows handling with switch and makes things somehow clearer.
If you want, i can send you a patch.