implement Tstat

drop extra do_send

fix typo. spotted by cage, thanks!

implement Twrite

allow directory to be read again using offset 0 The documentation says explicitly that the offset is *either* 0 or the last offset plus count; that means that after some read a client can require *again* an offset 0 (i.e. doing a rewinding the directory.)

implement Tread against directories To do so it was needed to modify the np_* functions so they take an explicit evbuffer.

PATH_MAX already accounts for NUL

free evp only if it was allocated apparently, evbuffer_free doesn't like NULLs

typo spotted by cage, thanks!

initialize evbuffer only for dirs

first half of the tread implementation done, no new tests yet

allow opening directories (for reading only)

fix walk fid check: look at fd not iomode to see if it was opened

ensure we don't call openat(2) with "" as path

fix Tattach handling The previous implementation assumed that you can't attach more than once. This is clearly wrong, Tattach and Twalk are the two ways to obtain new fids. This drops the error on subsequential attach and making the test "multiple attach" passes.