hx509: Add hx509.conf support

Just like krb5.conf, but hx509.conf, with all the same default locations
on Windows, OS X, and elsewhere, and HX509_CONFIG as the environment
variable equivalent of KRB5_CONFIG.

lib/hx509/hx_locl.h

@@ -208,6 +208,8 @@ struct hx509_context_data {
     struct et_list *et_list;
     char *querystat;
     hx509_certs default_trust_anchors;
+    heim_context hcontext;
+    heim_config_section *cf;
 };
 
 /* _hx509_calculate_path flag field */