Commit Briefs
cmd_ls: fix read size
don't use a constant: msize may be lower than that. Instead, use `msize - 4' which is guaranteed to be the maximum transferreable size. (it's not possible for msize to be lower than 4 since we reject ridiculously small msizes, so that difference can't underflow.)
allow Tread to fill a message
a message of `msize' len is valid, we just need to make sure to _not_ overflow it.
fail on "jumbo" requests (except for Twrite)
except for Twrite, all other requests are passed via a single imsg. We don't handle "jumbo" request for anything outside Twrite, nor it does make sense actually, so let's drop it. (well, for Twstat an argument can be made, maybe)