Drop setgid in favor of disabling process tracability explicitly (continue !2)

Initially on Phab as D23692, then imported into invent (!1 (closed)) and developed a bit further (!2 (closed)). This contains multiple commits from that MR:

Original review message from Malte Kraus:

So I noticed that kdesu is setgid 'nogroup'. That group is the fallback for groups from a remote NFS share that do not exist on the local machine. Since kdesu does not deal with NFS, I wanted to get rid of this (ab)use of 'nogroup'.

From all that I could gather (inline comments and a discussion on the KDE su handbook), the goal of the setgid bit on the binary is not to access any file as 'nogroup', but to prevent other processes of the calling user from accessing cached passwords, e.g. through ptrace(), core dumps or /proc//memory. While setgid is one way to achieve that, both Linux and FreeBSD allow setting a kernel flag to directly to disable such access. So I went for that.

reinstate KDEsuClient::isServerSGID for compat

prevent_tracing: make sure return var is initalized

prevent_tracing(): return success, improve warning text in main()