Add F_SETFD cmd to _fcntl function

[fjtrujy]

This PR basically makes the std::filesystem::directory_iterator work as expected.

I have used this example:

#include <stdio.h>
#include <filesystem>

int main(void)
{
    std::filesystem::directory_iterator dir_scanner("./");
    for (auto &entry : dir_scanner) {
        printf("%s\n", entry.path().string().c_str());
    }
    return 0;
}

It prints content of folder:

[    9.2121] dopen name host:/Users/fjtrujy/Downloads/filesystem-test-48641ffa 
[    9.2127] dopen fd = 256
[    9.2180] read/write allocate memory 4000
[    9.2182] ./CMakeFiles
[    9.2183] ./Makefile
[    9.2186] ./cmake_install.cmake
[    9.2231] ./filesystem-test.elf
[    9.2233] ./CMakeCache.txt

Cheers.

[uyjulian]

Lgtm. The POSIX.1-2008 calls can be implemented more fully in the future to make more libraries like libarchive working properly