std.os.linux: Implement clone3

[NicoElbers]

Implements clone3 based on the clone(2) man page.

I did my best to follow the convention that std.os.linux contains the straight syscall, and std.posix contains the syscall with error handling. If I broke some convention please let me know.

I did not add the ERESTARTNOINTR error case because I am unsure where exactly to add the errno code (only to linux, everywhere and if so where all is everywhere). If that's a prerequisite for merging please point me to where to add the errno code.

Finally in line with the other structs I found in std.os.linux I added minimal documentation to the fields and none to the flags. If I were to add the man page documentation it'd explode to a couple hundred lines which seems overkill.

Closes #18508

[alexrp]

Mm, there's overlap with #22368 here. I think it'd be ok to take the addition of the clone3 syscall wrapper separately from the main change in #22368, but you should probably coordinate with @ruihe774 then.

[NicoElbers]

This is an incorrect implementation, see #22368 (comment)