Unix-cred

Changelog:
- meta: improve efficiency of cross-platform interfaces on Solaris
- refactor: remove `getpeereid()`
- refactor(xucred): always define `Xucred.pid`
- fix(ffi): remove `None` check when loading libc (should fix on musl
libc)
- fix(ucred): use correct socket level on NetBSD
- meta: fix packaging issues
- doc: improved documentation
- test: improved tests

CHANGELOG:
- Use `None` instead of 0 if PID cannot be retrieved (BREAKING CHANGE)
- `xucred`: Store primary GID in separate field (instead of having it as a property that returns `self.groups[0]`)
- Don't load libc using ctypes on Linux, where it isn't needed (adds support for statically linked systems)

Initial release.

OS support:
- Linux (tested in CI)
- macOS (tested in CI)
- FreeBSD (tested in CI)
- DragonFlyBSD (untested)
- NetBSD (untested)
- OpenBSD (untested)
- Solaris/Illumos (untested; highly experimental)