Add time bindings

Fix forgotten ::size_t for freebsd

openbsd port

- 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

Add socketpair

Only run libc-test on nightly

Otherwise just make sure the library builds on other Rust versions

Add getaddrinfo functions

Add dladdr bindings

Add more sigset functions

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

Add some CLOEXEC constants

Add support to build as part of the standard library

Touch up some SA_* constants

Use bindings for -D_FILE_OFFSET_BITS=64

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

Add more bindings for signals and select/pselect

Define dirent_t on all platforms

Be more selective about skipped pthread on musl tests

Fixup the value of PTHREAD_STACK_MIN as well