vfs: add previous file version support for vfs_snapper

vfs: add previous file version support for vfs_snapper

Windows attempts to access previous file versions by first issuing an
FSCTL_SRV_ENUMERATE_SNAPSHOTS request, and then using the corresponding
snapshot timestamps in subsequent path based requests.

This change sees vfs_snapper provide snapshot enumeration data via the
get_shadow_copy_data VFS hook. Path based operations are also
intercepted, with @GMT-$time components converted to snapper .snapshot/#
paths.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>