aklog: Only try to use krb5-weak.conf if it exists

The logic we use for using krb5-weak.conf to allow 'weak crypto'
requires us to know where the default krb5.conf is. The default
krb5.conf local can vary significantly depending on the platform, and
we don't have a good way of figuring out what it is, so we guess. We
may guess wrong.

To limit the cases where we guess wrong, only try to do this
workaround if the krb5-weak.conf file actually exists.

Reviewed-on: http://gerrit.openafs.org/9667
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 95240a5a5030a1053ce9966b54037eda2d37140b)

Change-Id: I02313b2cc37da05adc0529e685abf6a8546bb3de
Reviewed-on: http://gerrit.openafs.org/9687
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>