Bug#1019426: openssh-client: ssh-agent can leave zombie ssh-pkcs11-help

Confirmed on openssh-client 1:9.4p1-1, ssh-agent seems not going to pick its dead children processes with wait(), no matter how they die. For example, running `ssh-add -s /usr/lib/x86_64-linux-gnu/opensc-pkcs11.so` with a smart card present will make ssh-agent spawn a ssh-pkcs11-helper to handle it, but the child will always become a zombie without being wait(2)ed after it dies, either terminating gracefully after running `ssh-add -e /usr/lib/x86_64-linux-gnu/opensc-pkcs11.so`, or being kill(2)ed.