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!

This adds extra checks to every t* function: if the client sent too much data per message, treat is an a protocol violation and terminate the connection. Regress suite still passes, this is just an extra safety check for finding wrong clients implementations. The twrite function was the only one with a check like this.

they should return 0 on failure, not -1

the fname field is just the file name, so NAME_MAX is more appropriate. Committing this separately from the rename change so it's easier to roll back eventually.

it's actually just the file name, not a path