scripts: Reworked dbgtag.py, added -i/--input, included hex in output

This just gives dbgtag.py a few more bells and whistles that may be
useful:

- Can now parse multiple tags from hex:

    $ ./scripts/dbgtag.py -x 71 01 01 01 12 02 02 02
    71 01 01 01    altrgt 0x101 w1 -1
    12 02 02 02    shrubdir w2 2

  Note this _does_ skip attached data, which risks some confusion but
  not skipping attached data will probably end up printing a bunch of
  garbage for most use cases:

    $ ./scripts/dbgtag.py -x 01 01 01 04 02 02 02 02 03 03 03 03
    01 01 01 04    gdelta 0x01 w1 4
    03 03 03 03    struct 0x03 w3 3

- Included hex in output. This is helpful for learning about the tag
  encoding and also helps identify tags when parsing multiple tags.

  I considered also included offsets, which might help with
  understanding attached data, but decided it would be too noisy. At
  some point you should probably jump to dbgrbyd.py anyways...

- Added -i/--input to read tags from a file. This is roughly the same as
  -x/--hex, but allows piping from other scripts:

    $ ./scripts/dbgcat.py disk -b4096 0 -n4,8 | ./scripts/dbgtag.py -i-
    80 03 00 08    magic 8

  Note this reads the entire file in before processing. We'd need to fit
  everything into RAM anyways to figure out padding.

scripts/dbgrbyd.py

@@ -162,7 +162,7 @@ def fromleb128(data):
 
 def fromtag(data):
     data = data.ljust(4, b'\0')
-    tag = (data[0] << 8) | data[1]
+    tag = struct.unpack('>H', data[:2])[0]
     weight, d = fromleb128(data[2:])
     size, d_ = fromleb128(data[2+d:])
     return tag>>15, tag&0x7fff, weight, size, 2+d+d_