Start gpg-agent in the "official" way to make ssh-support work.

I experience problems with ssh support of gpg-agent. The password management does not work, i.e. pinentry is not started and passwords are not remembered. Since it does work for others in #nixos I am doing something wrong, or at least different. The attached changes resolve the problem for me, so it seems that environment variables are not set correctly for me.

I will investigate further to see what causes the problem, to see if those changes are really needed.

My xinit/xsession file is located here: https://github.com/pSub/configs/blob/master/.xinitrc