This commit changes pf_readdir callback to its original behavior. Accesses and
streams now add items to a node.

Archive stream_filters will now be able to add nodes to a node (when an archive
has directory). This was not possible before.

This commit also adds an access_fsdir helper to help fs accesses (file, smb,
nfs, ftp, sftp) adding items to a node. These accesses need the same treatment
that is now done by this helper:
 - hide hidden files or not (depending on "show-hiddenfiles" option)
 - skip some file extensions (depending on "ignore-filetypes" option)
 - sort items by type and alphabetically (depending on "directory-sort"
   option).
 - For a next commit: attach slaves to items

The directory demux won't do these operations anymore for every access/stream.

This commit doesn't change the interruptible state of the pf_readdir function,
accesses/streams are still interruptible in the middle of a pf_readdir call.

This partially reverts commit 88ffe158.