Rename 'flags' to 'status' for struct metadata_area.

Rename 'flags' to 'status' for struct metadata_area.

In other LVM memory structures such as volume_group, the field
used to store flags is called "status", and on-disk fields are called
'flags', so rename the one inside metadata_area to be consistent.
Not only is it more consistent with existing code but is cleaner
to say "the status of this mda is ignored".

Background for this patch - prajnoha pinged me on IRC this morning
about a fix he was working on related to metadataignore when
metadata/dirs was set.  I was reviewing my patches from this year
and realized the 'flags' field was probably not the best choice
when I originally did the metadataignore patches.