Bug#394598: Some more info

[John Goerzen]

After quite some time, I discovered the problem to be
/var/spool/exim4/gnutls-params.

On my systems that hadn't been running exim4 before, but had freshly
installed this version, it was a happy ASCII text file.

On the system with the problem, it had binary data in it.

On a whim, I ran /usr/share/exim4/exim4_refresh_gnutls-params, which
solved the problem.

No idea why it was a problem, or why this fixed it.