[krbdev.mit.edu #2106] bug in krb5_cc_remove_cred API?

[Jeffrey Altman via RT]

The cc_remove field of every implementation of krb5_cc_ops is NULL. 
This is because there are no functions defined to implement the
cc_remove functionality.  This has been true since the very beginning. 
Clearly no one calls this function.

This can be fixed in one of two ways.  Add checks to the wrapper
functions to test for NULL and return an error if NULL is found; or add
the functions to each of the implementations and have the
implementations return an error.

My preference is to add functions to each of the implementations.  The
hardest question to answer is which error should be returned.   I do not
believe any of the existing error values is appropriate and a new error:
KRB5_CC_NOSUPP should be added.