kamid: simplify "not a directory" check

fix compilation: struct np_stat is defined in kami.h

- [regress-extra] added test for checking truncation of files using Twstat.

work around wstat limitation 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!

fail if the received packet is too long 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.

fix np_read{8,16,32,64} functions they should return 0 on failure, not -1

kamid: change fid fname size to NAME_MAX 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.

kamid: rename field fpath to fname it's actually just the file name, not a path

fmt

ftp: document rename

fix ls: don't hardcode fid=1

ftp: add nextfid to find the next lower free fid

add `rename' command it leverages the recently added wstat support

9pclib: add wstat support

fmt