PLUGIN: gpg-agent: export SSH_* environment variables too

If using the gpg-agent with --enable-ssh-support, the SSH_AUTH_SOCK and
SSH_AGENT_PID environment variables need to be exported once sourced
from GPG_ENV.  Otherwise, we get no benefit from the persisting these
values to GPG_ENV; subsequent openned terminals don't see the existent
gpg-agent as a process for an SSH daemon.