vfs_snapper: encode and decode Snapper DBus strings

vfs_snapper: encode and decode Snapper DBus strings

Snapper uses a special character encoding for strings used in DBus
requests and responses. This change ensures that Samba packs and unpacks
strings in the corresponding format, using the previously added
encode/decode helper functions.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11055

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
(cherry picked from commit 32e9d7fa220d05f5ca95ed61cf6c7aa7d0261c03)