The idea is to abstract over the users that kamid manages. The virtual table allows to use a custom userbase instead of the local UNIX users. The userdata tables maps (virtual) users to their home directory, which is the only directory tree exported by kamid as of now. Together, they allow a powerful and flexible management of the users and their data. The idea is somewhat stolen from smtpd(8).

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

Implement Topen plus some basic testing. ORCLOSE (remove file when the fid is clunked) is mapped to O_CLOEXEC and tried to be honoured on fid_free. "vanilla" 9P2000 uses reads on directories to list the entries while 9P2000.L (and .U too possibly) introduces an explicit Treaddir. I'm planning to support 9P2000-style read-on-dir but not yet.

sizeof(struct np_msg_header) includes unwanted padding...