FreeBSD error numbers for Rust

This library defines an Error struct that represents error numbers returned from FreeBSD system calls.

To depend on freebsd-errno from a Bazel workspace:

```python load("@bazeltools//tools/builddefs/repo:http.bzl", "http_archive")

httparchive( name = "rustposixerrno", sha256 = "0c86c849ff673372fe6415d4004a233565b57b2884ea49d3b725dd1296cc2529", stripprefix = "posix-errno-1.0.1", urls = ["https://github.com/jmillikin/rust-posix-errno/releases/download/v1.0.1/posix-errno-1.0.1.tar.xz"], )

httparchive( name = "rustfreebsderrno", # Obtain the package checksum from the release page: # https://github.com/jmillikin/rust-freebsd-errno/releases/tag/v1.0.0 sha256 = "", stripprefix = "freebsd-errno-1.0.0", urls = ["https://github.com/jmillikin/rust-freebsd-errno/releases/download/v1.0.0/freebsd-errno-1.0.0.tar.xz"], ) ```

To depend on freebsd-errno from a Cargo workspace:

[dependencies] freebsd-errno = { version = "1.0.0" }