Opening files without metadata EA may result in broken metadata EA

Opening files without metadata EA may result in broken metadata EA

ad_open_hf_ea() calls ad_flush() after initializing the new metadata,
but in ad_flush() ad_flush_ea() wasn't called because we didn't
yet increment the refcount.
Fix this by increasing the refcount in ad_open_hf() before calling
into ad_open_hf_ea(), decrementing it again in case of an error.

This ensure a valid metadata EA is created, but it still doesn't
set the CNID field values. Therefor in afp_openfork() we must check
whether the ad_open() created a metadata EA and then set the CNID
info.

Finally, check for malformed metadata EAs and delete them.

Fixes bug #510.