lib/krb5: Align with MIT behaviour and provide krb5_init_creds_opt_set_fast_ccache...

lib/krb5: Align with MIT behaviour and provide krb5_init_creds_opt_set_fast_ccache() and krb5_init_creds_opt_set_fast_flags()

It is easier for external callers to manipulate the krb5_get_init_creds_opt
(via the helpers) as this is passed down from higher up than the krb5_init_creds_context.

And just as importantly, alignment with MIT makes end-user callers happier.

Finally, this resolves the ambiguity as to which layer owns the
krb5_ccache, because now we match the MIT behaviour the init_creds code
re-opens a private copy inside libkrb5, meaning the caller closes the
cache it opened, rather than handing it over to the library.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>