Make ddb ssh/login pass SSH_AUTH_SOCK through.

[mfeit-internet2]

Pass SSH_AUTH_SOCK through to the shell process if:

• It exists
• It points into one of the shared volumes (/tmp, /run' or what was in $USER_HOME on boot)
• The socket it points to exists

Should also have prep install ssh.

[mfeit-internet2]

Problem: Sharing /tmp in from the host causes problems for programs running in the container.