agent socket path can easily get too long

See

  http://comments.gmane.org/gmane.comp.encryption.gpg.devel/21073

When the path to the agent's socket is > 107 bytes long, it exceeds the kernel's(?) size limit, and gpg-agent doesn't work.

This can quite easily happen in testsuites that create a fake home directory and point GNUPGHOME into it. For example, this affects devscripts and libgnupg-interface-perl currently. For the production autopkgtest infrastructure, this only happens on s390x because that happens to use the lxc backend to autopkgtest which creates slightly longer paths for its home directories.