Swap out posix calls for standard equivalent

Posix calls are not portable and when they are, they spew are lot of warnings about deprecation. Swap them out for equivalent standard library implementations.

Also, avoid features like `fork` in implementation since there is no equivalent for it on non-unix like platforms. Prefer explicit threads instead.

Search the C++ implementation for POSIX calls and fork usage, then identify the affected portability boundaries and existing threading patterns. Done means the relevant calls use standard-library equivalents, fork is avoided in the implementation, and the project still builds on Linux, Windows, and macOS.

Written by the indexing model from the issue text.