Improve Unix support #53
Description
Currently, many of the data sources used to make the library work are Linux specific, and are typically compiled away on Unix because of the #[cfg(target_os = "linux")] macro.
As we talk about Unix a lot in the README and offer Unix support more broadly, once we have discussed https://github.com/appsignal/appsignal-agent/issues/629, we should add Unix counterparts for the data sources/commands used in this library.