add warning about suid/sgid binaries to new injector

We should update the docs and the new default early injector to warn about suid/sgid binaries, which we will fail to inject into (and the app will fail to run, which if it's a child inside a larger process tree will likely fail the whole parent app). We can still suggest the /etc/ld.so.preload workaround, by keeping libdrpreload.so around.