slim: drop CAP_SYS_PTRACE from bounding set
This matches the systemd service definition. I've tested this so it is ready to merge if desired.
However, I wonder if it is desired? I wonder why this particular hardening was selected for slim? It limits what you can do when sudo'd to root within a terminal, e.g. nsenter -a -t $(cat /etc/service/exim4/supervise/pid)
won't work.