[krbdev.mit.edu #3039] Extend krb5_ccache to support plugable implementations

[kenh@cmf.nrl.navy.mil via RT]

>Perhaps in krb5.conf a new section:
>
>[krb5_ccache-plugins]
>PREFIX1 = library-path1
>PREFIX2 = library-path2

It would be nice if this feature could be turned off at compile time
(I know, this is rather an odd request).  We provide kits for a number
of architectures, and I'd prefer to NOT give users the ability to
add an arbitrary credential cache type.

--Ken