Clarify/determine specification for padding bytes, (formerly also PADDING cell)

EDIT: strikethrough content below is now covered in #26870 (moved) instead

==== Background I was trying to interpret the tor-spec for padding bytes, and ending up asking nickm for some clarification over IRC. nickm suggested most of the cc'd for the ticket - I added atagar, too.

==== Unclear areas Here are the points that need clarification / specification:

• spec for padding bytes does not clearly say what senders MUST or SHOULD do, [mentioning that padding is with 0 bytes] or [NUL bytes]
• spec for padding bytes does not say what receivers MUST or SHOULD do, when receiving non-zero bytes in the Cell (e.g. warn? ignore?)
• spec is a bit inconsistent with PADDING cells ^^[1^^]^^[2^^]

==== Discussion: padding bytes For the padding bytes that are not part of PADDING cells, nickm offered the following as a non-exhaustive set of possible forward-compatible options:

• "the [padding] bytes SHOULD be zero, and that implementations MUST ignore them"
• "The first 8 padding bytes MUST be zero; all subsequent padding bytes SHOULD be randomized. Implementations MUST ignore padding bytes"
• "All padding bytes should be randomized; implemenations MUST ignore unrecognized padding bytes" ... and mentioned that "[he doesn't] know enough of the argument in favor of randomization to have a very strong preference"

==== Inconsistency: PADDING cell payload (see bullet above)

These references highlight the inconsistency:

^^[1^^] PADDING: Payload is unused. per [3 "Cell Packet format"]. ~~ implies 0 bytes of payload, so the rest should be padded per that section~~ ^^[2^^] The contents of a PADDING, VPADDING, or DROP cell SHOULD be chosen randomly, and MUST be ignored. per [7.2 "Link padding"]. ~~ implies the payload of a PADDING cell actually is the rest of the size of the cell, and that it SHOULD be chosen randomly~~

The PADDING cells were mentioned in IRC but not discussed. I think a simple change to make the spec consistent between the two sections would be this: PADDING: Payload contains random data. (See Sec 7.2)

However, given the other points here, is that correct?