Patch looks mostly good. Some comments:

- sethostname supports names with embedded null bytes, so
  wrapper should not use strlen
- it's a bit asymmetric that gethostname is in the socket
  module and supports Windows, and sethostname is in the POSIX
  module. It would be useful to have a gethostname in the POSIX
  module also which is a) POSIX only and b) supports embedded
  NUL bytes.
- I think get/sethostname should use the FSDefault encoding.
- gethostid should check for POSIX errors.
- it checks for sethostid but then doesn't implement it.
- parsing id_t with "l" is incorrect, methinks
- siginfo_t should map to a structsequence
- according to my copy of waitid(2), not all of the fields you
  are returning from siginfo_t are actually filled.
- instead of PARSE_OFF_T, I rather recommend to use an O& parser.