Thanks for the comments.

I implemented sethostid() - its not actually in the posix spec, but linux/*bsd
have it although the signature for FreeBSD is a bit different.

I implemented gethostname(). Both get/sethostname() now use FSDefault encoding.
gethostname() is null-terminated so it doesn't have to deal with embedded NULs.
sethostname() works with embedded NULs.

Since id_t can contain a pid_t, I now parse id_t with _Py_PARSE_PID.
waitid() also now returns a structsequence of the correct fields.

According to the spec, gethostid does not set errno - it now checks anyway.

Instead of PARSE_OFF_T, I use a O& parser.