vfs_streams_xattr: add options "prefix" and "store_stream_type"

vfs_streams_xattr: add options "prefix" and "store_stream_type"

Add module options that can be used to configure the stream prefix the
module uses (option "prefix", a string) and whether the stream type
"$DATA" is appended to the xattr name on disk (option
"store_stream_type", a boolean).

The default "prefix" is "user.DosStream" and the default for
"store_stream_type" is true, this gives unchanged default behaviour
when not specifying this option.

OS X SMB clients will send xattrs as named streams over the wire, by
setting the options to the following values

  streams_xattr:prefix = user.
  streams_xattr:store_stream_type = no

OS X xattrs will be stored on disk on the server with their unmodified
names and as such provide interoperability with other protocols like
AFP.

In order to prevent access to our internal Samba xattrs, check the
xattr name with the function samba_private_attr_name() made public by
the previous commit.

Signed-off-by: Ralph Boehme <rb@sernet.de>
Reviewed-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>