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

[Jeffrey Altman via RT]

The krb5_ccache implementation is very close to a pluggable interface.
It uses a prefix name (API, FILE, MSLSA, etc) as an index into a table 
of function pointers.   By adding a configuration file parameter, sites 
could develop their own ccache implementations without requiring
re-compilation of the krb5 libraries.

Perhaps in krb5.conf a new section:

[krb5_ccache-plugins]
PREFIX1 = library-path1
PREFIX2 = library-path2