[krbdev.mit.edu #6803] Config variable for default ccache directory

[Ken Raeburn via RT]

Implementing this change would also be a good time to allow setting of the default ccache type -- or the default name-including-type, which is different -- so that a system can be switched to default to keyring ccaches, for example.

A tougher one would be coming up with a mechanism to recommend for sshd, pam, login, and the like to use to generate a new ccache name with a system-wide default type specified in a config file, which may have different type-specific ways of incorporating pty names, session ids, randomly generated strings, etc.  Currently, I believe changing the default ccache type for a system means hacking multiple remote-access programs separately?

Ken