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

it leverages the recently added wstat support

it's not exactly compliant, as 9p requires the changes to be done atomically, but it's not possible to better targeting a UNIX file system. The 9p manpage regarding wstat isn't clear about the behaviour when changing the name of the file: kamid changes the associated fid to reflect the rename. kamid also allows to rename files to other directories, the fid is updated to the new location even in this case.