client: pxar: opt encode cli exclude patterns as Prelude

client: pxar: opt encode cli exclude patterns as Prelude

Instead of encoding the pxar cli exclude patterns as regular file
within the root directory of an archive, store this information
directly after the pxar format version entry in the entry of kind
Prelude.

This behavior is however currently exclusive to the archives written
with format version 2 in a split metadata and payload case.

This is a breaking change for the encoding of new cli exclude
parameters. Any new exclude parameter will not be added to an already
present .pxar-cliexclude file, and it will not be created if not
present.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>