Use POSIX file API directly instead of stdio FILE

[mackron]

When compiling with -std=c89, builds will fail due to functions like fdopen() not being available. Uncomment the section at the top of fs.c and compile with FS_FORCE_C89 in the CMake script to replicate the errors. Compile with GCC.

Once this is done the Windows backend can be implemented with native Win32 APIs and we can discard the stdio stuff completely.