env KRB5CCNAME=/tmp/foocc kinit ignores the env

The problem is that fcc_get_cache_next() is called in a context where
context->default_cc_name is not set.  We should call
krb5_cc_default_name(), and that fixes the problem.  There's a comment
warning that this can result in reentering krb5_cc_cache_match(), but
nothing in libkrb5 calls krb5_cc_cache_match(), so the comment is wrong,
at least in the github tree.

An alternative would be to call krb5_cc_set_default_name(NULL) in
kuser/kinit.c before calling krb5_cc_cache_match(), however, that seems
like an insufficiently general solution.  Also, the semantics of
krb5_cc_cache_match() would differ from MIT's -- it seems better to
match MIT's semantics.