Calls to internal functions from client plugins

[Sam Hartman]

Keep in mind that client side plugins cannot generally depend on being
able to call functions from k5-int.h.

On all platforms we're trying to tighten up the export list and remove
private symbols.  On Windows and Mac we have a fairly tight export
list and would not add symbols in order for plugins to call them.

Instead, take a look at the krb5int_accessor function in k5-int.h for
a mechansim for getting to call internal functions.

--Sam