Fix ubuntu bug 1302638

Opening this PR as requested by Raphaël Pinson in https://bugs.launchpad.net/ubuntu/+source/augeas/+bug/1302638
This is my first time doing that, so if I'm doing it wrong, please correct me.

When building, at the linking stage I run into this issue: https://github.com/hercules-team/augeas/issues/88
My topic branch `fix-YYID_undefined_reference' trivially works around that. I should 've bisected the tree, but haven't the time. I suspect it may actually be an autotools issue?

This fix-ubuntu-bug-1302638 makes the krb5.aug lens process lower case realms in the [realms] section, but not in the [appdefaults]. So there are still valid krb5.conf that the lense will not parse. This seems to be by design: in order to be able to label an entry 'app' or 'realm' in the appdefaults section.
I see only ways out of this situation that break backward compatibility.

Thanks for reviewing!
Jurjen