Feature: Add f_process project.

Feature: Add f_process project.

This provides pidfd functionality that I intend to utilize in future work.

Provide `f_handle_t` for represetning `struct file_handle`.

Several of the functions being represented are not in some libc implementations.
Use `syscall()` to handle these.
Provide defines for enabling/disabling the `syscall()` behavior.
Provide stubs for mocking such functions in unit tests so as to avoid trying to mock `syscall()` itself.