twstat was serializing the qid in the wrong order. While here, add a comment before the qid definition too.

The idea behind woc_file is to create or open a file, but it's always used for writing fresh new content, so it's actually an error that it opens a file for writing without truncating it too. It means that if we write less content than what the file already has, we kepp garbage at the end. This adds an extra argument in send_fid (that is actually used only from woc_file now) so that we can pass KOTRUNC for do_open.

found by clang' static-analyzer

This is the counterpart for the recent kamid commit about wstat: we're lacking the leading size field.

9p stat message have a leading 2 bytes long size field. It's meant to simplify the parsing, because while reading the contents of a directory it's necessary to know how much long an entry is, so the length field is present even in the Rstat reply and in the Twstat input. Previously we didn't consume that bit and thus mis-read all of the following fields.

the previous commit added an explicit check for extra data in messages. The problem is that wstat voluntarily skips reading some fields at the end, so the len will always be greater than zero!