[krbdev.mit.edu #2657] krb5_init_context() calls into the CCAPI via krb5_cc_set_default_name()

[Alexandra Ellwood via RT]

Currently krb5_init_context() calls into CCAPI when it resets the ccache name in the context.  
It does this by calling krb5_cc_set_default_name(context, NULL).

krb5_cc_set_default_name(context, NULL) sets the ccache name in the context to its "default" 
value by calling get_from_os().  On CCAPI platforms it calls 
cc_context_get_default_ccache_name().

The problem is that cc_context_get_default_ccache_name() produces an IPC call.  This means 
that you pay for the overhead of an IPC call just for initializing a context.  It also means that 
the CCAPI server needs to be present to answer the request, even though the context may 
never use the ccache.

I would like to change the behavior so that it lazily looks up the default ccache.  Basically 
when you call krb5_cc_set_default_name(context, NULL), it will just set the ccache name in 
the context to the NULL string and free the old one.  When you call 
krb5_cc_default_name(context), it will look at the environment variable and then call 
get_from_os().  Basically I just want to defer the actual work to when the library needs to look 
up the ccache name.

Although this changes when the default ccache is looked up, I can't think of any practical 
problems with it based on how callers use krb5_contexts