nits in filexfer-06

[Jacob Nevins <jacobn+secsh%chiark.greenend.org.uk@localhost>]

draft-ietf-secsh-filexfer-06.txt says:

| 3.  General Packet Format
| 
|    All packets transmitted over the secure connection are of the
|    following format:
| 
|        uint32           length
|        byte             type
|        uint32           request-id
|            ... type specific fields...
|        byte[length] data payload
| 
|    'length' is the length of the entire packet, excluding the length
|    field itself, such that, for example, for a packet type containing no
|    type specific fields, the length field would be 5, and 9 bytes of
|    data would be sent on the wire.  (This is the packet format used in
|    the secsh transport. [2]

This is wrong in two respects, and confusing in several others:
 * The last line of the packet description should be
     byte[length-5]   data payload
   to match the rest of the language.
 * Two packets, SSH_FXP_INIT and SSH_FXP_VERSION, have no `request-id'
   field. I think it's easier to leave it out of the "General Packet
   Format", although it's probably worth mentioning that `request-id'
   exists in most packets in this section.
 * The inclusion of "... type specific fields ..." above this can be
   read to imply that there might sometimes be extra fields _between_
   the "request-id" and "data payload". I presume the "type specific
   fields" is actually intended to refer to the "data payload" field
   itself.
 * I think "This is the packet format used in the secsh transport"
   could also be confusing. I'd go for something more like "This is a
   similar convention to that used in the secsh transport packet
   format".

I don't see much wrong with the language in filexfer-05 and previous,
TBH.