Add clone3 to permitted system calls

[thedanbob]

I’m on Arch Linux and my latest system update broke doh-client. I got an error message saying it couldn’t spawn a thread, permission denied. I’m not sure exactly what changed but I figured out that adding clone3 to the system call filter fixes it.

Edit: for reference, I believe this was the change that caused the problem: https://github.com/archlinux/svntogit-packages/commit/14abe44a37dedc67d11563122502b62896ecf94f clone3 used to be disabled in glibc on Arch but was just re-enabled.