xattrs: introduce a rsync_xa_list struct as layer between two nested item_lists

xattrs: introduce a rsync_xa_list struct as layer between two nested item_lists

We have the global 'item_list rsync_xal_l', this maintains an array
of rsync_xa_list structure, one per file.

Each rsync_xa_list structure maintains an array of rsync_xa structure,
while each represent a single xattr with name and value.

Part of a patchset for bug 5324.