Patchset on krb5 ccache handling

[fihuer]

This patchset is about making auks agnostic to the underlying ccache type. This touches different parts of auks:

• The auks C API by adding some helper function for ccache management (generation/destruction)
• Aukspriv should not directly manage ccaches, instead it now relies on kinit to do the job
• The Spank plugin, same as aukspriv, it should not directly manage ccaches but only use the api

Fixes #11 May fix #23

[hautreux]

Let's try it