vfs_snapper: add DBus string encoding and decoding helpers

vfs_snapper: add DBus string encoding and decoding helpers

Snapper uses the following mechanism for encoding and decoding strings
used in DBus traffic:
Characters above 127 (0x7F - ASCII DEL) must be encoded hexadecimal as
"\x??". As a consequence "\" must be encoded as "\\".

This change adds string encoding and decoding helpers to vfs_snapper.

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 3d48fc96739bde631f8197aa313a81808481adf3)