Update for `redox_syscall` 0.3, and removed system calls

If we want to avoid libc, perhaps https://gitlab.redox-os.org/redox-os/relibc/-/tree/master/src/platform/redox/redox-exec could be published on crates.io, or integrated into redox_syscall. But just calling into libc and letting relibc handle it is generally fine.

Things like fork() are problematic from a safety standpoint if one isn't careful (generally you don't want to do anything like allocation between fork and exec), and the way this is handled could use some auditing.

But in any case, this make strace work again.