Recover from errors in ancillary chunks

As per the third edition of the spec, decoders should recover from
errors in ancillary chunks.

In section 13.1 Error handling [1]:
> Anomalous situations other than syntax errors shall be treated as
> follows:
>  1. Encountering an unknown ancillary chunk is never an error. The
>     chunk can simply be ignored.

More specifically, in this commit, if a chunk that is detected as
ancillary, does not pass the `check_chunk_name()` function, only a
_benign_ is issued, instead of an error.

This allows libpng to fully decode images like [2] and [3]. It has been
tested by passing them to both pngtest and Gnome's image viewer. Note
that invalid-unknown-ancillary-after-IDAT.png could already be displayed
but not fully decoded.

[1] https://w3c.github.io/png/#13Decoders.Errors
[2] https://github.com/web-platform-tests/wpt/blob/master/png/errors/support/invalid-unknown-ancillary.png
[3] https://github.com/web-platform-tests/wpt/blob/master/png/errors/support/invalid-unknown-ancillary-after-IDAT.png