move symbolic link for certificates

This is my first Pull Request, so apologise any errors.

I propose to move the symbolic link in {prefix}/share from port globus-clients to igtf-certificates.
The background is that it is useful for any software which would rely on GSI authentication. I have written ports for UberFTP and iRods (with GSI) support. There depend on the library, the proxy-tools but not necessary on the other globus-clients.