implement mknodat

[zmlcc]

This adds the mknodat function, which is part of POSIX https://pubs.opengroup.org/onlinepubs/9699919799/functions/mknod.html

mknodat seems not implemented in OSX, so I add conditional compilation attributes to exclude ios and macos

I don't find any test code about mknod, So tests of mknodat are not added either

[asomers]

Yes, mknod may not have any test code, and that's a shame. You should still add tests for mknodat, and while you're at it it would be great to add them for mknod as well.

[zmlcc]

mknod test failed in many arch/os. Is mknod not widely implemented in different systems ?

[asomers]

The tests are probably failing due to permissions errors. Creating a device node requires root.