Refactor: use functionality based checking instead of OS-level checking

[tskaar]

What?

This refactors some of the code to use functionality based checking rather than OS-level checking for header includes

Motivation

For bleeding edge systems that are using GLIBC v2.42, termio.h has been removed, and termios.h is the replacement. Due to recent refactors + removal of these headers one is not able to compile this when using GLIBC v2.24.

Note

Only tested the compilation on my machine (Arch Linux x86_64), hopefully this doesn't break for other systems (BSD etc.)!

Closes: #434