Fortran filesystem

add functions:

• space_available(): tells drive space available for this specified path
• is_char_device(): is a path a special character device like a terminal, /dev/null
• is_reserved(): for Windows, is a path reserved like NUL, COM1, etc. (always false on non-windows)

For Windows C-only internals, use COMSPEC env var instead of cmd.exe directory for create_directories() mkdir as a security best practice, inspired by Python 3.11.3 subprocess.

add test_reserved for all functions to ensure predictable behavior for /dev/null, NUL, etc. With regard to fs_equivalent/same_file, we treat any non-directory, non-regular-file as always unequal (like NaN or Infinity in IEEE754)

fs_is_unix() is now also true for macOS

copy_file(): error handling for case source file does not exist for stability/robustness