- remove multiple definition of stack_t
- make types resolv correctly
- correct target definitions
  - dragonfly is already defined as freebsdlike
  - bitrig is an openbsdlike
- imports should be public

Not gonna gain much from issuing warnings about various aspects! This is managed
separately from the standard library

They're all added under the `foo64` names with the `foo64` types added as well,
so they still need to be explicitly chosen.

Fixup the value of PTHREAD_STACK_MIN as well