[Libguestfs] [PATCH 1/2] options: Fixes and enhancements to --key parsing.

The first patch fixes a rather serious bug, the second patch allows multiple --key parameters and default parameters. There is a third patch to libguestfs which adds a test, coming up. I did not yet review and fix the documentation. I think we need to centralize it in one place because at the moment the same documentation for --key is copy/pasted all over the tools. Rich.