sg: remove `ulimit -n` from cmds, set on boot in `sg`

Created by: mrnugget

Instead of running ulimit -n per process and probably ending up forgetting to run it in some of them (as was the case with zoekt here in Slack), we can use a syscall to set the maximum number of open files once sg starts up.

That's harmless since the limit only persists for the lifetime of the process and it's quick too.