Add explicit flags to override Fil-C resource paths instead of relying
solely on directory discovery relative to binary location:

- --filc-resource-dir: Override pizfix root directory
- --filc-dynamic-linker: Override ld-yolo-x86_64.so path
- --filc-crt-path: Override CRT objects directory
- --filc-stdfil-include: Override Fil-C runtime headers
- --filc-os-include: Override kernel headers
- --filc-include: Override libc headers

These flags take precedence over automatic discovery but maintain
backwards compatibility with existing pizfix and /opt/fil setups.

This allows build systems like Nix to use wrappers with explicit
paths without requiring specific directory layouts or copying
resources into pizfix structures.

Amp-Thread-ID: https://ampcode.com/threads/T-5bd1dd1a-c537-49c3-9b23-1b4e27ce4e81
Co-authored-by: Amp <[email protected]>

When --filc-crt-path or --filc-resource-dir is passed, enable Fil-C
mode even if pizfix directory doesn't exist. This allows Nix wrappers
to fully control paths via flags without directory discovery.