Avoid simple memory dumps via ptrace

This avoids needing to setgid gpg-agent. It probably doesn't defend
against all possible attacks, but it defends against one specific (and
easy) one. If there are other protections we should do them too.

This will make it slightly harder to debug the agent because the
normal user won't be able to attach gdb to it directly while it runs.

The remaining options for debugging are:

• launch the agent from gdb directly
• connect gdb to a running agent as the superuser

Upstream bug: https://dev.gnupg.org/T1211

Gbp-Pq: Topic block-ptrace-on-secret-daemons
Gbp-Pq: Name Avoid-simple-memory-dumps-via-ptrace.patch