add chunk inspection to pb-debug

add chunk inspection to pb-debug

Adds possibility to inspect chunks and find indexes that reference the
chunk. Options:
 - chunk: path to the chunk file
 - [opt] decode: path to a file or to stdout(-), if specified, the
   chunk will be decoded into the specified location
 - [opt] digest: needed when searching for references, if set, it will
   be used for verification when decoding
 - [opt] keyfile: path to a keyfile, needed if decode is specified and
   the data was encrypted
 - [opt] reference-filter: path in which indexes that reference the
   chunk should be searched, can be a group, snapshot or the whole
   datastore, if not specified no references will be searched
 - [default=true] use-filename-as-digest: use chunk-filename as digest,
   if no digest is specified

Signed-off-by: Hannes Laimer <h.laimer@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>